Add admin feature to curate user accounts

[markmacgillivray]

It may be useful to have frontend superuser control over user accounts. Superuser can do anything at the moment (which would include deleting a specific user account) but has not got an overview UI of all user accounts.

[markmacgillivray]

There is a /users route now which shows all users, but there is an error on bibsoup.net - fix that then this will be doable by the superuser