R1:621f806e3b54

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 {nav Config > HTTP Parameter Types}.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132…