Don't require display name during email sign up

[samtstern]

This is a mirror of firebase/FirebaseUI-Android#467

We should increase the flexibility of email sign up by:

• Not making display name a required field
• Allowing the developer to easily customize the label on the display name field

[samtstern]

cc @yramanchuk

[samtstern]

Web issue: firebase/firebaseui-web#41

[aleczadikian]

Any resolution here? Maybe I'm missing something, but I don't see an obvious way to disable the first and last name field in email/password signup.

[MrDrProfX]

I would LOVE the ability to disable first and last name from the firebase UI.