Set default subject in e-mails

Fixes #600.

app/src/main/java/org/fdroid/fdroid/AppDetails.java

@@ -1080,7 +1080,8 @@ public class AppDetails extends AppCompatActivity {
                         url = app.webURL;
                         break;
                     case R.id.email:
-                        url = "mailto:" + app.email;
+                        final String subject = Uri.encode(getString(R.string.app_details_subject, app.name));
+                        url = "mailto:" + app.email + "?subject=" + subject;
                         break;
                     case R.id.source:
                         url = app.sourceURL;

app/src/main/res/values/donottranslate.xml

@@ -13,6 +13,8 @@
     <string name="https">https://</string>
     <string name="upgrade_from_to">%1$s → %2$s</string>
 
+    <string name="app_details_subject">%1$s on F-Droid</string>
+
     <string-array name="updateIntervalValues">
         <item>0</item>
         <item>1</item>