Fix #249: Never cache CSRFToken.
Fix #248: Fix next param logic and move logic to Python.

By deciding whether to redirect the user to the next param URL or the
default URL in Python instead of JavaScript, we can rely on Django’s
is_safe_url logic.
Fix #243: Document removal of context processor.

Also adds a small reminder to Troubleshooting about leftover pyc files.
Fix the admin integration by adding a missing browserid_info tag.
Fix #231: Add error handling to JS and document the API a bit better.
Fix #253: Use stateless Persona API instead of workarounds.