Use a shorter and less surprising session key
.env is supposed to be ignored
Ensure that all ENV vars are set if needed
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.
Split out admin features
Store settings in the database
Use Setting for review period open/closed
Allow admins to open/close review period via UI
Update README for new admin web UI features
Extract admin login in features
Let admins send intro/closure mails via web

The emails are sent using ActiveJob (though this is not configured to do
any background processing yet).

We're using activejob_backport as Rails 4.2.0 is not yet released, and
not all of our gems are compatible with the beta.
Add confirm pop-ups on significant admin actions
Make admin log out button fit in header
Move localisation of pages to separate file
Localise admin UI
Localise user create/edit buttons