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.

git subrepo pull vendor/registripe

Also update vendored_requirements to make sure we pull this in.

subrepo:
subdir: "vendor/registripe"
merged: "9fc3645"
upstream:
origin: "git@gitlab.com:tchaypo/registrasion-stripe.git"
branch: "lca2018"
commit: "9fc3645"
git-subrepo:
version: "0.3.1"
origin: "???"
commit: "???"