[UX] Change the username field on registration to "Display name"

Related: - Hide the registration username field if users can only login with email - We should not allow users to use email address as username

We should start transitioning to email as the unique identifier for logins instead of usernames.

But we use usernames all over our sites. We can begin by making it clear that the "username" is how your name will appear when Backdrop is referring to your profile. Eventually we can transition to Profiles or Name fields.