Update to Django 2.2

Upgrade site and modules to Django 2.2. Remove and replace obsolete
functionality with current equivalents. Update requirements to latest
versions where possible. Remove unused dependencies.

Require login for proposal submit

- This has the effect of bouncing people to the login page if they're
unauthenticated, rather than returning a 502 because 'home' doesn't
exist.

- If they're authenticated but don't have a speaker profile, send them
to the speaker profile create page rather than just to the
dashboard.

Closes #26

Taking one out of PyCon's (US) book

We're lock-step with symposion, and upstream is dead.
Vendor it.