Give user an option to select between Facebook and Gravatar picture

[mitar]

When linking will be possible (#76), we should add to option for users to select which user picture they want displayed: Gravatar or Facebook. If user first registered through Facebook, the default should be Facebook, if user first registered directly, then after linking default should change to Facebook if user does not have a Gravatar picture. (Is there a way to test this?)

Blocked by: #76

[mitar]

Authentication has moved to separate package, so this should be implemented there: mitar/django-mongo-auth#6