Remove unique constraint on resetPasswordToken

If this column is required to be unique then there can't be two entries
in the accounts collection with a 'null' resetPasswordToken, which means
there is an error as soon as a second person tries to sign up.
Redirect to new instance page after registration

After a user has registered for an account take them to the new instance
screen.
Add link to user admin on instance app

Link to the user admin page from the top menu in the instance app if
they have permission to access the admin page.
Add link to registration from login screen