Fill in new URI credential edit web UI interfaces

Summary:
Ref T10748. Ref T10366. Allows users to set credential for new URIs.

• Ref T7221. Our handling of the "git://" protocol is currently incorrect. This protocol is not authenticated, but is considered an SSH protocol. In the new UI, it is considered an anonymous/unauthenticated protocol instead.
• Ref T10241. This fixes the PassphraseCredentialControl so it doesn't silently edit the value if the current value is not visible to you and/or not valid.

Test Plan:
Performed a whole lot of credential edits, removals, and adjustments. I'll give this additional vetting before cutting over to it.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7221, T10241, T10366, T10748

Differential Revision: https://secure.phabricator.com/D15829