Make vendored symposion into an installable Python package

This allows us to install with `pip install "-e vendor/symposion"` similar to
the other vendored packages. There's no good reason for this to be different to
the others and depend on PYTHONPATH hacking.

Re-add

Switch vendored requirements to "editable" to simplify development

This will allow us to remove the `sys.path` hacking in pinaxcon/settings.py.

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 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: "???"