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.

Add an all_products property to FlagBase

It's common to need to query the fill list of products covered by a
Flag - whether directly, or by being in an included category.

Add an all_products property which does this.

git subrepo pull vendor/registrasion

subrepo:
subdir: "vendor/registrasion"
merged: "c1e194a"
upstream:
origin: "git@gitlab.com:tchaypo/registrasion.git"
branch: "lca2018"
commit: "c1e194a"
git-subrepo:
version: "0.3.1"
origin: "???"
commit: "???"