remove gmail-oauth2

- add a device message for existing profiles potentially using gmail
- reset oauth2 method for existing profiles on opening login screen
- do not offer gmail-oauth2 for new profiles

src/main/java/com/b44t/messenger/DcContext.java

@@ -260,6 +260,15 @@ public class DcContext {
       }
     }
 
+    public boolean isGmailOauth2Addr(String addr) {
+      final String oauth2url = getOauth2Url(addr, "chat.delta:/foo");
+      return isGmailOauth2Url(oauth2url);
+    }
+
+    public boolean isGmailOauth2Url(String oauth2url) {
+      return oauth2url.startsWith("https://accounts.google.com/");
+    }
+
     /**
      * @return true if at least one chat has location streaming enabled
      */