OC Avatars

:wrench: WIP :wrench:

## OC Avatars
Here they are (sort of).

---
TODO:
 - [ ] Use custom default avatars @kabum 
 - [ ] Finish designing @jancborchardt 
 - [ ] Make the gravatar override-button work
 - [ ] Implement custom backends
 - [ ] Fix unittests (encryption and local avatars @schiesbn @FlorinPeter)
 - [ ] Get rid of all `TODO`s


Review the code!