(stable) Flatten "RemarkupValue" objects when setting field defaults for custom forms

Summary:
Ref T13685. "RemarkupEditField" uses a "RemarkupValue" internally, and it currently attempts to serialize into the database unsuccessfully.

Instead, flatten the value before returning it for storage.

Test Plan:

• Edited the default description of a task in a custom form.

Maniphest Tasks: T13685

Differential Revision: https://secure.phabricator.com/D21854