Provide formal Users/Projects/Mailable fields for EditEngine

Summary: Ref T9132. This allows you to prefill EditEngine forms with stuff like ?subscribers=epriestley, and we'll figure out what you mean.

Test Plan:

• Did /?subscribers=... with various values (good, bad, mis-capitalized).
• Did /?projects=... with various values (good, bad, mis-capitalized).
• Reviewed documentation.
• Reviewed Config → HTTP Parameter Types.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14404