Stats for likes

 Adds two more stats, top liked messages and top liked users, top liked users only takes into consideration liked messages (content_type == 'msg').

Might want to check the queries for getting the stats. Perhaps they could be improved.

I also encounter a token validation error while saving both mentions and likes settings, after some debugging it turns out the inline permissions were causing it, init_inline_permissions() needed to create the token that save_inline_permissions() validates, creating said token inside  init_inline_permissions() appers to fix the issue but more testing would be needed.
The "background" property is a shortcut that allows you to specify all the individual properties on a single line. This is no different than this:
```
background-image: url(../images/generic_icons.png);
background-repeat: no-repeat;
background-position: -64px 0;
```