Context manager for selecting content DB

[jamalex]

Summary

Adds a context manager/decorator to select content databases across a given set of database operations.

TODO

• Have tests been written for the new code? Some, but more coverage still needed
• Has documentation been written/updated? Inline docs
• New dependencies (if any) added to requirements file

[jamalex]

This is the file where most of the work is done.

[rtibbles]

@jamalex There were some weird conflicts here, as I think I had rebased my branch. Cherry picked it in, but unfortunately stole the blame for one of the commits in the merge conflict resolution :/

Is in my branch now.

[jamalex]

Oh, ok -- I did some rebasing against your js_api branch, but I guess you rebased again after that. ⚾ As long as it all makes it in!

Note on this stuff: it works for most cases. One problem (which I largely mitigated, but might still surface) is that Django's DB routing gets called at the time a queryset is resolved, not the time it's created, so if a queryset is created inside the context manager and then returned to elsewhere before lazy resolution occurs, it's possible it will throw an error. I think the cases in which this would happen should be rare now, with some changes I made and through upgrading to the bleeding edge version of django-mptt that includes this fix from a week ago: django-mptt/django-mptt#478

[rtibbles]

Yeah - I have further mitigated this issue by removing any mention of specific databases within the content API methods - so now they have to be called from within a context manager.

The one case where this has still cropped up is in the Django REST framework Serializers, where a queryset is returned from methods and then further processing done. Eli has worked around this by continuing to use the .using method in this context.