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

https://jira.ets.berkeley.edu/jira/browse/CYB-321
CYB-321 - Provide UI support for generic URL submission
CYB-321 - Added default thumbnail for submissions without a thumbnail image
CYB-321 - Removed transparent border around the default thumbnail
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

https://jira.ets.berkeley.edu/jira/browse/CYB-321
CYB-321 - Improved default thumbnail for gallery items without thumbnail
CYB-321 - Swapped default thumbnail png for svg
Merge branch 'generic_url_processing' of https://github.com/coyote/datacultures into generic_url_processing
Merge branch 'generic_url_processing' into generic_url_processing_ui

Conflicts:
	app/assets/templates/gallery_list.html
	spec/fixtures/generic_urls/generic_urls.json
Merge branch 'generic_url_processing' of https://github.com/coyote/datacultures into generic_url_processing
Merge branch 'generic_url_processing' into generic_url_processing_ui
Merge branch 'generic_url_processing' into generic_url_processing_ui
Merge branch 'master' of https://github.com/ets-berkeley-edu/datacultures into generic_url_processing_ui
Merge branch 'master' into generic_url_processing_ui
CYB-321 - Self review
CYB-321 - Removed empty g tags from SVG
Merge pull request #223 from nicolaasmatthijs/generic_url_processing_ui

CYB-321 - Provide UI support for generic URL submission