Expose user avatar URL field in the UI

[jeena]

To be able to change a avatar the URL field needs to be exposed but changing the URL alone is difficult. This patch adds a image upload field to the edit view of a user. If there is a file detected it uploads it to the servers media repository and sets that new URL as the avatar URL one. This way a admin can change any users avatar which is especially nice for bridged users like in the IRC bridge and bots.

Fixes #60