Save tile size with changed name
Don't provide a default value for image dimensions

All Images in the database now have a 'dimensions' field. If a new Image
without a sensible 'dimensions' field is saved, a validation exception
should rightfully be raised. Providing a nonsense "[0,0,0]" default value
is not a good thing, as it may disguise bugs where 'dimensions' isn't
being set.
Merge branch 'image_dimension'

* image_dimension:
  Don't provide a default value for image dimensions