Refactor so that `openedx_content` is a top-level package

[ormsbee]

After #457 is complete, refactor so that the openedx_content app is a top level package in openedx-learning, rather than something inside of openedx_learning.apps. This work will also require that all openedx-platform references to these paths be updated.

It's also worth noting that the openedx_learning.lib package has modules that are used by openedx-platform for things like case_insensitive_char_field, in order to normalize behavior between SQLite and MySQL (e.g. for test consistency). I'm not clear on where these should go, since they're not really tied to openedx_content.

I think it's likely we'll want to use the openedx_learning name for another top level app that will have some Learning Pathways related code in it.