Add support for door code management

[anaulin]

We need to change our support of door codes for our new space.

The new workflow will look like:

1. Admin generates X (unused) codes in the app. These codes are marked as "not yet available for use" (i.e. they have not yet been entered in the actual door).
2. Admin enters the unused codes in the door, and marks them as "entered in the door, but not assigned to anyone" in the app.
3. Available codes can then be assigned to members in one of two ways:
   1. A member submits the key-member form. The app auto-assigns them an existing available code, and marks the code as assigned.
   2. An admin can manually assign a member an existing code.

In addition, admins should be able to manually enter door codes and statuses in the app, as well as manually assign codes to members.

This issue might need to be broken up into individual issues (especially if multiple people want to split the work)

To do:

• wipe everybody's "key member" status from the database (one-time manual operation)
• wipe the contents of the existing door_codes table
• add a status column to the door_codes table; the initial set of available statuses can be: not_yet_in_lock, in_lock, assigned, blacklisted (blacklisted status is to prevent users from using easily-guessable pin codes, see Prevent easily-guessed pin codes #593)
• verify that codes need to be at least 6-digit, and the autogenerated ones are 7-digit long
• ensure that only numerical codes are allowed
• add admin UI for generating new door codes in the "not in lock" state
• add admin UI for changing the status of an existing code
• add admin UI for assigning an un-assigned code to a member
• add admin UI for entering codes manually, together with a status
• show both name and email in user list dropdown in code edit form; sort names alphabetically; only show members in list
• update list of existing door codes to be partitioned by status, and sort alphabetically by "assigned to" name within each section
• remove unnecessary key-code / key-member related bits from "Manage Members" view
• make key membership form submission auto-assign an existing code to the member
  • if there are no available codes, email the board and let the member know