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.

Create regidesk app

Shows summary of all attendees with a paid ticket, including
boarding_pass status.

Currently, regidesk allows staff with the requisite permission the
ability to view the checkin status of attendees, and email the user
their boarding pass email.

Included is a view for the user to retrieve their own QR code (in case
they got the plain-text version of the email, they can use this to
download an image to their phone for faster checkin)

git subrepo clone git@gitlab.com:LCA2018/registrasion-desk.git vendor/regidesk

subrepo:
subdir: "vendor/regidesk"
merged: "a8096d0"
upstream:
origin: "git@gitlab.com:LCA2018/registrasion-desk.git"
branch: "master"
commit: "a8096d0"
git-subrepo:
version: "0.3.1"
origin: "???"
commit: "???"