streamline help menu

- use clear "Delta Chat Homepage" for opening our homepage
  (instead of unclear "Learn more about Delta Chat",
  which is esp. weird, when used from inside the help)

- do not "promote" GitHub in our menu entry,
  instead of "Contribute on GitHub", just say "Contribute".
  destination URL is our contribute page then -
  that is also more useful for ppl doing eg. translations

src/main/java/org/thoughtcrime/securesms/LocalHelpActivity.java

@@ -68,7 +68,7 @@ public boolean onOptionsItemSelected(MenuItem item) {
         openOnlineUrl("https://delta.chat/gdpr");
         return true;
       case R.id.contribute:
-        openOnlineUrl("https://github.com/deltachat/deltachat-android");
+        openOnlineUrl("https://delta.chat/contribute");
         return true;
       case R.id.report_issue:
         openOnlineUrl("https://github.com/deltachat/deltachat-android/issues");

src/main/res/menu/local_help.xml

@@ -5,15 +5,15 @@
     <item android:title="@string/menu_scroll_to_top"
         android:id="@+id/log_scroll_up" />
 
-    <item android:title="@string/global_menu_help_learn_desktop"
+    <item android:title="@string/delta_chat_homepage"
         android:id="@+id/learn_more"
         app:showAsAction="never"/>
 
     <item android:title="@string/privacy_policy"
         android:id="@+id/privacy_policy"
         app:showAsAction="never"/>
 
-    <item android:title="@string/global_menu_help_contribute_desktop"
+    <item android:title="@string/contribute"
         android:id="@+id/contribute"
         app:showAsAction="never"/>
 

src/main/res/values/strings.xml

@@ -1086,8 +1086,12 @@
     <string name="global_menu_view_developer_desktop">Developer</string>
     <string name="global_menu_view_developer_tools_desktop">Developer Tools</string>
     <string name="global_menu_help_desktop">Help</string>
+    <!-- deprecated, use delta_chat_homepage instead -->
     <string name="global_menu_help_learn_desktop">Learn more about Delta Chat</string>
+    <string name="delta_chat_homepage">Delta Chat Homepage</string>
+    <!-- deprecated, use contribute instead -->
     <string name="global_menu_help_contribute_desktop">Contribute on GitHub</string>
+    <string name="contribute">Contribute</string>
     <string name="global_menu_help_report_desktop">Report an Issue</string>
     <string name="global_menu_help_about_desktop">About Delta Chat</string>
     <string name="global_menu_file_open_desktop">Open Delta Chat</string>