Enhance user profile: allows users to upload an avatar (issue #230).

The display_user_avatar() helper displays in that order:

* The user's uploaded avatar.
* The user's Twitter avatar if the previous one does not exist
* Nothing if no avatar is available (maybe a placeholder should be interesting, I'll add an issue for this when I have some network connection).

TODO: we need to find out how to test file uploads for both the profile and resources controllers.
Fixes specs on user controller (linked to issue #230)

I had to commit first to stash many changes without losing my feature. Then I was able to run specs.
Merge branch 'master' into feature_8.0