Improve default subject on post replies

Test on one device of your choice. Visit a forum discussion containing replies with custom and default subjects. Write a reply to existing replies editing the default subject. Check subject of the reply is shown. Write a reply to existing replies but do not edit the default subject. Check subject of the reply is hidden Assert that the default subject for the new replies depend on the parent subject, not the discussion subject. Assert that not more than one "Re: " prefix is prepended for nested reply subjects. Start writing a reply without modifying the subject. Without submitting the reply, start replying to another post. The subject should have been updated. Change the reply subject. Without submitting the reply, start replying to another post. The subject should not have been updated. Switch between nested and flat replies mode to make sure that the same subjects are kept displayed and hidden.