Allow block developers to easily indicate that the app can render the content returned by WS

For some type of blocks, those that are very simple, it would be good to allow developers to just indicate some where in the db/mobile.php file that the app can just render the content returned by the blocks WS (with an additional recommendation about implementing the method "get_contents_for_external" to retrieve a more friendly app view)

We already do this for the HTML block and some other blocks in the app.