Retrieve Gravatars by mime-type

Old avatars will eventually be purged by the delete method
Removed magic string
Merge pull request #1571 from BarryThePenguin/avatar-extensions

Retrieve Gravatars by mime-type Fixes #1467
ui avatars: Gravatar also supports GIFs
avatars: Fix redownloading of avatar every time
repo: Fix crash due to last timestamp being empty when adding a new empty project