Fix twitter image URL thumbnails in DB

[fxneumann]

Hi,

the current version generates the right link to pic.twitter Thumbnails, but the links already in the DB don't get fixed.

I used this query to repair it (assuming it's always 50 chars, as it was in my DB):

update `tn_tweets` set `extra` = replace(`extra`,'50:"http://p.twimg','52:"http://pbs.twimg');

This could be placed in upgrade.php.