Epic: add support for user-defined collections of websites

[alicewriteswrongs]

As a course author, I'd like to write a list of courses similar to this one: https://ocw.mit.edu/courses/energy-courses/

To support this, we want to add support for defining a 'collection' of websites. This will let a content author add a new list and add websites to it, using our existing APIs to build out an editing UI.

These collections can then be added as embedded content to our WYSIWYG editor, which will allow content authors to publish pages like the one linked above (for instance in the ocw-www 'entry' site).

Acceptance Criteria:

• add Collection model #315
• add serializers for updating WebsiteCollection and WebsiteCollectionItem models #334
• Add views for fetching and editing Collections #316
• add an index page for viewing WebsiteCollections #356
• tab view for courses, collections #371
• add UI for editing / creating WebsiteCollections #357
• add support for website collections to the WYSIWYG editor #317
• permissions for WebsiteCollections and WebsiteCollectionItems #355
• UI for adding new websites to a website collection #376
• Add ability to remove courses from WebsiteCollection #396

[alicewriteswrongs]

@Ferdi @pdpinch quick question, should a site content author be able to add a course to a WebsiteCollection more than once?

[alicewriteswrongs]

This epic concerned our earlier implementation of the feature - I'm going to close this so we don't get confused (or so I don't get confused anyway!)