Added Gmail web OAuth flow

Thanks @cketti for pointing in the right direction!

app/src/amazon/AndroidManifest.xml

@@ -465,6 +465,15 @@
 
                 <data android:scheme="eu.faircode.email" />
             </intent-filter>
+
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+
+                <data android:scheme="eu.faircode.email.debug" />
+            </intent-filter>
         </activity>
 
         <service

app/src/fdroid/AndroidManifest.xml

@@ -469,6 +469,15 @@
 
                 <data android:scheme="eu.faircode.email" />
             </intent-filter>
+
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+
+                <data android:scheme="eu.faircode.email.debug" />
+            </intent-filter>
         </activity>
 
         <service

app/src/github/AndroidManifest.xml

@@ -469,6 +469,15 @@
 
                 <data android:scheme="eu.faircode.email" />
             </intent-filter>
+
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+
+                <data android:scheme="eu.faircode.email.debug" />
+            </intent-filter>
         </activity>
 
         <service

app/src/main/AndroidManifest.xml

@@ -463,6 +463,15 @@
 
                 <data android:scheme="eu.faircode.email" />
             </intent-filter>
+
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+
+                <data android:scheme="eu.faircode.email.debug" />
+            </intent-filter>
         </activity>
 
         <service

app/src/main/java/eu/faircode/email/FragmentOAuth.java

@@ -403,12 +403,19 @@ public class FragmentOAuth extends FragmentBase {
                     ? new LinkedHashMap<>()
                     : provider.oauth.parameters);
 
+            String clientId = provider.oauth.clientId;
+            Uri redirectUri = Uri.parse(provider.oauth.redirectUri);
+            if ("gmail".equals(id) && BuildConfig.DEBUG) {
+                clientId = "803253368361-hr8kelm53hqodj7c6brdjeb2ctn5jg3p.apps.googleusercontent.com";
+                redirectUri = Uri.parse("eu.faircode.email.debug:/");
+            }
+
             AuthorizationRequest.Builder authRequestBuilder =
                     new AuthorizationRequest.Builder(
                             serviceConfig,
-                            provider.oauth.clientId,
+                            clientId,
                             ResponseTypeValues.CODE,
-                            Uri.parse(provider.oauth.redirectUri))
+                            redirectUri)
                             .setScopes(provider.oauth.scopes)
                             .setState(provider.id)
                             .setAdditionalParameters(params);

app/src/main/res/layout/fragment_gmail.xml

@@ -165,6 +165,7 @@
             android:text="@string/title_setup_gmail_on_device"
             android:textAppearance="@style/TextAppearance.AppCompat.Small"
             android:textColor="?android:attr/textColorLink"
+            android:visibility="gone"
             app:layout_constraintEnd_toEndOf="parent"
             app:layout_constraintStart_toStartOf="parent"
             app:layout_constraintTop_toBottomOf="@id/tvSelectHint" />

app/src/main/res/xml/providers.xml

@@ -23,15 +23,15 @@
         <oauth
             askAccount="true"
             authorizationEndpoint="https://accounts.google.com/o/oauth2/auth"
-            clientId="495454862802-rkeb9fs6i6aqs99ieureo5nugc5g98lc.apps.googleusercontent.com"
-            clientSecret="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"
-            enabled="false"
+            clientId="803253368361-11ias0ee6bqhvdi4f21fs1lh7fsb0il2.apps.googleusercontent.com"
+            enabled="true"
             privacy="https://policies.google.com/privacy"
             prompt="consent"
-            redirectUri="https://oauth.faircode.eu/"
+            redirectUri="eu.faircode.email:/"
             scopes="https://mail.google.com/"
             tokenEndpoint="https://oauth2.googleapis.com/token">
             <!-- https://developers.google.com/identity/protocols/oauth2/native-app -->
+            <!-- https://github.com/openid/AppAuth-Android/blob/master/app/README-Google.md -->
             <parameter
                 key="access_type"
                 value="offline" />

app/src/play/AndroidManifest.xml

@@ -465,6 +465,15 @@
 
                 <data android:scheme="eu.faircode.email" />
             </intent-filter>
+
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+
+                <data android:scheme="eu.faircode.email.debug" />
+            </intent-filter>
         </activity>
 
         <service