Summary:
Ref T9132. This allows you to prefill custom fields with `?custom.x.y=value`, for most types of custom fields.

Dates (which are substantially more complicated) aren't supported. I'll just do those once the dust settles. Other types should work, I think.

Test Plan:
  - Verified custom fields appear on "HTTP Parameters" help UI.
  - Used `?x=y` to prefill custom fields on edit form.
  - Performed various normal edits.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14634