Surface custom form instructions as a "Remarkup" field for the transaction layer

Summary: Ref T13263. See https://discourse.phabricator-community.org/t/image-uploads-for-forms-too-restricted-by-default/2594. Currently, when you add instructions to a custom form, we don't expose them as remarkup, so {Fxxx} references don't get attached correctly and won't get proper permissions.

Test Plan:
Dragged-and-dropped an image into form instructions, saw the file attach to the form:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13263

Differential Revision: https://secure.phabricator.com/D20387