[DEV-1739] Re-enable 'login/signup to post'

It turns out that the reason this was broken was that Askbot had hooked it into its forked django-authopenid.

Shun. Shun. Shhhhuuuuunnnnnnn.

I reimplemented it by changing the unique identifier on Questions/Answers posted while logged out from a session key to a uuid, and redirecting from login to a view that looks up the content by its uuid, posts it, then redirects to it.

Note that I added a comment and a bit of custom code at the very top of the migration script, because it doesn't make sense to run this migration forwards or backwards if there are records in the tables being migrated.