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