permissions for WebsiteCollections and WebsiteCollectionItems

[alicewriteswrongs]

In #349 we put a pretty simple permissions setup in place - basically, all users in the GLOBAL_ADMIN and GLOBAL_EDITOR groups get full access to create, update, etc our website collections.

We should figure out exactly what permissions setup we want for this. A few possibilities:

• should there be more groups, COLLECTION_ADMIN and COLLECTION_EDITOR?
• should all editors have the ability to create new collections?
• what about adding elements to them?

etc.

[pdpinch]

Currently, web site editors who don't have COLLECTION_ADMIN cannot see the list of courses in a related widget, for example, when editing a course collection.