CYB-223 Refactor the likes API

* 'canvas_scoring_item_id' field on Activity renamed 'scoring_item_id' as it now can also refer to DataCultures local items as well
* lib/array_refinement.rb#video_hash changed to use string keys, to keep the same structure as #image_hash
* GET api/v1/gallery implemented and test (spec/controllers/api/v1/gallery_controller_spec.rb)
* POST api/v1/likes implemented (in app/controllers/api/v1/likes_controller.rb)

Most of the changes relate to the field rename.

[CYB-223 Refactor the likes API](https://jira.ets.berkeley.edu/jira/browse/CYB-223)