Set max width 100% for images

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

@@ -465,6 +465,8 @@ public class HtmlEx {
                         out.append(((ImageSpan) style[j]).getSource());
                         out.append("\"");
 
+                        out.append("style=\"max-width: 100%; height: auto;\"");
+
                         if (style[j] instanceof ImageSpanEx) {
                             ImageSpanEx img = (ImageSpanEx) style[j];
                             int w = img.getWidth();