Support versions and proxies in collections

We want the ability to store versions (and maybe proxies) in collections.

In order to do this, we'll need to be able to add facets to versions and modify the properties of the facet's schemas even though the version is read-only. This capability will be reserved for special schemas deemed "system" enough so that even though the version is technically modified, it's not a content change so the modification is acceptable. In the context of collections, the schema (CollectionMember) that we use is akin to a relation.