Explicitly set height for #page-footer contents.

This should prevent anything below the footer, at least unless it wraps, which
should be prevented by the explicit font-size and width.

Fixes
https://trello.com/c/jlkmVAKo/168-bug-logo-in-der-leiste-abgeschnitten-chrome
Allow trusted userprofiles to have any avatar in the avatar directory.

We have to validate UserProfile->image ourselves in clean(), as it is dependent
on other values of the model, namely the trusted flag.
Don't overwrite the UserProfile->image field when getting ""

This fixes the bug which resets the user's avatar when editing the profile if it
is not one of the freely available ones.