extra APT sources needs to be a fixed list to chose from, with restrictions

We need to change extra APT sources to be a fixed list; some entries are only allowed for a given set of users.

This should be a configuration file. YAML is probably the best format for it.

- id: some-identifier
  entry: deb http://...
- id: restricted-entry
  entry: deb http://...
  allowed_users: [122132, 4252] # id

• id is the field that can be used in the request
• entry is what actually gets passed to autopkgtest
• allowed_users, if present, restricts which users can use that entry. if not present, any user can use it.

in the list of users, id is the primary key of the users table. not username, not uid (non-human users like britney don't have uid's from salsa)

in the UI, add a message saying that new archives can be added by talking to the admins