Option for login username

Currently the username field on the login page can be the username or the email. This should be configurable for a realm where the options are:

• Username and Email
• Email only
• Username only

This will affect the label on the login form, how users are looked-up as well as account management console. For email only the username field would not be needed. For username only the email would not be needed and we could purely rely on a contact email field instead.