Publishing Entity Types, refnames, and namespacing container/component identifiers

[ormsbee]

(placeholder note to continue the discussion started in #495 (comment))

[ormsbee]

In addition to the comments already in the ContainerTypes PR, one of the things @kdmccormick and I discussed was the possibility of entirely decoupling the publishable entity refname from the namespaced identifier that components need. We could also take that even further and make the namespace/type/code identifier lookup M:1. For example, if we wanted to have a single PublishableEntity that was both a Video XBlock and an Asset, we could have:

• PublishableEntityType that encodes (namespace, type, subtype?)
• PublishableEntityLookup that has a fkey to PublishableEntityType, a code, and an fkey to PublishableEntity.

With that arrangement, said hypothetical video block could be both ('xblock.v1', 'video', 'intro_video'), as well as ('asset', 'video', 'intro_video') at the same time—we could have these represented as two PublishableEntityLookup rows that point to the same PublishableEntity.

(This is a very hypothetical example, and I'm not sure if we'd want it. But I think it's feasible.)

[ormsbee]

Tangential thought: this kind of lookup and aliasing functionality could be useful for third party integrations in general.

[kdmccormick]

fyi @bradenmacdonald