R1:c1ae5321d745

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.