Password managers will try to guess what they should save as a username
by looking at the fields on the page where you set up your password.
When registering from an invite the email address (what we use as a
username) is predefined, and only shown on the page as text, not an
input.
This commit also adds a hidden input field for password managers to pick
up.
Adapted from: https://github.com/UKGovernmentBEIS/beis-opss-psd/blob/master/app/views/users/complete_registration.html.erb#L29-L36
834b0fc3d5