Implement a "credential" standard custom field

Summary: Ref T4590. Ref T1049. This is primarily intended to support HTTP auth in Harbormaster.

Test Plan: Added a field, edited it, etc.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4590, T1049

Differential Revision: https://secure.phabricator.com/D8607