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.

Improve slot editing

Fix placement of form to allow submit button to work again.
Display information about slot on the edit popup.

Don't duplicate existing rooms when importing schedule CSV

* Old implementation needs to see exactly the same rooms in exactly
the same order every time it loads new data, otherwise it will
create a duplicate entry for the room that differs only in display
order.
* New implementation ignores the display order when checking to see
if the room already exists.

Wrap input CSV in an TextIOWrapper

Closes #24

Taking one out of PyCon's (US) book

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