Provide drop-down menu of roles when adding user

Resolves #57

Create Add User admin function that has a drop-down menu of possible
roles rather than requiring the admin to type a valid role in
correctly.

Remove a number of hard-coded references to different controller
types, especially the Table/Report (and, for now, Admin-Table)
controllers that work with settings files.

Change locking_key from integer to character string since some tables
(including ramp_auth_users) do not have integer keys.