Flatten "RemarkupValue" objects when setting field defaults for custom forms
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.
- Edited the default description of a task in a custom form.
Maniphest Tasks: T13685
Differential Revision: https://secure.phabricator.com/D21854