Redirect back to instance after account upgrade

When an account is upgraded redirect the user back to the instance which
they were originally trying to log into.
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
Legacy API credentials will continue to work for now

Add a note to the legacy account upgrade screen informing the user that
their legacy credentials will continue to work for the time being.
Add link to registration on the homepage
fixup! Redirect to new instance page after registration