CYB-321 Use Canvas' thumbnailing for non-YT/Vimeo URLs

* Adds a new model, GenericUrl
* Adds queries for #index and #show actions of the GalleryController

[CYB-321  Use Canvas' thumbnailing for non-YT/Vimeo URLs](https://jira.ets.berkeley.edu/jira/browse/CYB-321)
Since there is no explicit #initialize for a call to .new, I use the no argument style (rather than empty), as that gives a misleading impression of the method signature.