Add the ability to log in as an administrator

* A user can be an administrator
* It's possible to log in using an Identity
* Administrators can log out
* There is an admin home page with nothing on it
Show tabs when we want them

… instead of hiding them when we don't.
Let admins upload a CSV of users
Change pluralisation for consistency

Change UsersImporter to UserImporter for consistency with UserUpload
(because UsersUpload would imply UsersUploadsController which seems like
madness).
Add user CRUD for admins

There's also a script in bin/create_admin that will create (or update)
an admin login and print the password to the terminal.