Use canvas by default (not webGL)
Stack and comparison do not consider canvas when webGL is not available.
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
Don't use a leading dot in URLs

All of the changed URLs should be absolute resource paths.
Use absolute paths for tiles in session editor

The session editor is now at a RESTful location, so relative paths to
tiles won't work.
A sesssion's 'hideAnnotations' field should be a bool

The old code allowed it to be treated as an int in some cases, and past
commits didn't fully account for this.
Merge branch 'session_editor_fix'

* session_editor_fix:
  A sesssion's 'hideAnnotations' field should be a bool
  Use absolute paths for tiles in session editor
  Don't use a leading dot in URLs
WIP
No need to delet if session not found
Merge branch 'fix/resync'

* fix/resync:
  No need to delet if session not found
Merge remote-tracking branch 'github/master' into dev/dj-searchbox

* github/master:
  No need to delet if session not found
  A sesssion's 'hideAnnotations' field should be a bool
  Use absolute paths for tiles in session editor
  Don't use a leading dot in URLs
  Create all model mutable default values using a callable
  Make 'Image.dimensions' a required field
  Fix a bug in Facebook login
  Stack and comparison do not consider canvas when webGL is not available.
  Use canvas by default (not webGL)
Directives magic