Expand admin ui for checkins to be useful for admins

... also (and this really should be another commit, I am a horrible
person) change name of exception text fields

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: "???"