Remove Individual Items by moving to Redis

Closes https://github.com/skalnik/print-queue/issues/7

To do:
* [x] Add a UID for each queue item (`hash(url, email, timestamp)`) :snowflake:
* [x] Move to Redis
* [ ] Update docs w/Redis info
* [x] Add a single queue item delete route for admins :walking: :walking: :x: :walking:
* [x] Hook it up to the UI :sparkles:
* [ ] Remove `/tmp` if we don't need it anymore