Add "Posts" tab in admin area

[tvbeek]

A list of "posts" sorted on updated_at to see the last changed posts (and the last created will be there because then created_at = updated_at). This can be useful for detecting spam messages.

I should think about something like a table with:
| user | thread| content| updated_at (updated yes/no)|

Where user and thread are linked to the corresponding user and thread.

This can be replies but also threads start itself but I think starting with replies will already help a lot.