Exclude rel elements for favicons

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

@@ -111,6 +111,10 @@ public class ContactInfo {
     private static final long CACHE_GRAVATAR_DURATION = 2 * 60 * 60 * 1000L; // milliseconds
     private static final long CACHE_FAVICON_DURATION = 2 * 7 * 24 * 60 * 60 * 1000L; // milliseconds
 
+    private static final List<String> REL_EXCLUDE = Collections.unmodifiableList(Arrays.asList(
+            "dns-prefetch", "preconnect", "prefetch", "preload", "prerender"
+    ));
+
     private ContactInfo() {
     }
 
@@ -551,7 +555,8 @@ public class ContactInfo {
         List<Future<Bitmap>> futures = new ArrayList<>();
         for (Element img : imgs) {
             // https://developer.mozilla.org/en-US/docs/Web/Performance/dns-prefetch
-            if ("dns-prefetch".equals(img.attr("rel"))) // gmx.net
+            String rel = img.attr("rel").trim().toLowerCase(Locale.ROOT);
+            if (REL_EXCLUDE.contains(rel)) // dns-prefetch: gmx.net
                 continue;
 
             String favicon = ("link".equals(img.tagName())