Ensure that `openedx_content` cannot create a corrupted data state

[kdmccormick]

There are ways in which you can call the authoring_api that leads the DB to be broken. For example, before we fixed it, you were able to:

• Create a container like a Unit
• Create a new ContainerVersion, but NOT a new UnitVersion
• Now, app code will see the ContainerVersion and assume it has a .unit_version, causing a 500
• There's no way out of this state without modifying MySQL yourself

AC: Fix the specific issue listed above, if it's not fixed already.

• Note: Is there anything more general we want to do about the PE<->PEV relationships that would avoid these sort of inconsistencies? Is there any sort of metadata we'd want to add to capture things like "Y is a subtype of X, XV is a version of X, and YV is a version of Y... so if Y exists and XV exists then YV should exist".

AC: Scan through authoring_api and ensure that any "invalid" operation raises an exception rather than putting the DB in a bad state.

[bradenmacdonald]

The simplified data models I'm proposing at #412 (comment) would solve the example problem here, by eliminating UnitVersion.