add admin migration
80308648: Added pagination to the items index.
* Requested changes from pr review
* added tests for pagination
* moved tests for item pagination into user_opens_app feature tests
* rearranged the step definitions to align with primary use
82451782: Artists can now have one associated image.
Changed association between image and item to be polymorphic.
Added model tests to confirm that items and artists can access their
image(s).
Tested by hand in the UI to confirm that item images still work.
Fixed the migration to fill in the imageable_type.