Fix a bug in Facebook login

Only 'GroupRole' objects may have a 'facebook_id' field.
Merge branch 'facebook_group_fix'

* facebook_group_fix:
  Fix a bug in Facebook login
Make 'Image.dimensions' a required field

This removes all references to a possible 'dimension' field.

The database will be updated to ensure all Image documents have a
'dimensions' field and that all 'dimension' fields are renamed to
'dimensions'. Non-updated code will not break, as it already assumes that
either of the two fields is acceptible.
Create all model mutable default values using a callable

MongoEngine default values are similar to Python function default
arguments, in that only a single instance of the default value is created
when the MongoEngine field is first defined. Subsequent requests for the
default value would return the same mutable object, unless it's a callable
that creates a new mutable object on demand.
Merge branch 'image_dimensions'

* image_dimensions:
  Create all model mutable default values using a callable
  Make 'Image.dimensions' a required field