Create registration / new user shell

[oubiwann]

Tasks:

• [x] support telnet access
• [x] identify all required commands
• [x] create help file for all commands
• [x] receive and store data in the session:
  • [x] session id
  • [x] email address
  • [x] ssh public key
  • [x] confirmation status
• [x] ensure that all commands that can work with session data do so correctly
• [x] update service_user schema (add new/missing fields)
• [x] extract DB errors and selectively display to users (e.g., if email address is already taken) - #21
• [x] verify user accounts - #22
• [x] store all session data in the DB - #23
• [x] Keep user account table clean - #24

Blocks: #11

[oubiwann]

Current prototype needs to be refactored along the lines outlined by Fred in LYSE.

[oubiwann]

I'm going to put the registration supervisor under the top-level mush supervisor.

[oubiwann]

Here's something that could be used for TTL'ing unconfirmed users after 24 hours:

• https://stackoverflow.com/a/26063344

[oubiwann]

Sub-tasks and tickets are complete!