Revert and Reimagine Link to last read posts

Revert http://tracker.phpbb.com/browse/PHPBB3-11969

This implementation goes against all tradition in phpBB and any other user forum software.

The issue was the link icon to the last unread post was too small in mobile devices.

The solution was wrong.

Topic titles should always link to the first topic of the the thread.

Topic post dates should not be links either (that was just done as no-alternative fallback).

The correct solution should be to restore original functionality to Prosilver.

Then we can use the bigger icon topic row icons to provided direct access to first unread posts, which will also work well when the responsive design shrinks down to a mobile size.