R1:1a52c8f713cc

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:

{F6367710}

Reviewers…