Add "allow null" and username hinting to the Passphrase credential control

Summary:
Ref T4122.

• For Diffusion, we need "allow null" (permits selection of "No Credential") for anonymous HTTP repositories.
• For Diffusion, we can make things a little easier to configure by prefilling the username.

Test Plan: Used UIExample form. These featuers are used in a future revision.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4122

Differential Revision: https://secure.phabricator.com/D7624