refactor: Units inherit Containers (Multi-Table Inheritance)

[bradenmacdonald]

This PR modifies openedx#278 to use multi-table inheritance, so Units are a subclass of Containers.

Features:

• Unit in python isinstance of Container ✅
• UnitVersion in python isinstance of ContainerVersion ✅
• Unit.versioning.draft will give you a UnitVersion, and Container.versioning.draft will give you a ContainerVersion ✅
• There is only one foreign key to PublishableEntityVersion (on ContainerVersion, not on UnitVersion) ✅
• Unit and UnitVersion have their own database tables and can be the source/target for unit-specific foreign keys ✅
• Actually has fewer queries than the original version for most operations ✅
• Models like Unit and UnitVersion are now so simple that we don't need a ContainerMixin nor ContainerVersionMixin at all ✅
• How we store metadata is left undecided but this PR maintains complete flexibility.