Support HTTP parameter prefilling in EditEngine forms for CustomFields

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