Recognize remarkup custom fields as remarkup blocks

Summary:
Fixes T5916. Key insight here is that the screenshot shows a custom "Detail Solution / Notes" field, which is why this mojo doesn't work: custom remarkup fields don't emit their content for mention/file extraction.

Also fix a bug where multiple blocks with file PHIDs could be merged improperly, discarding some file PHIDs.

Test Plan: Added a custom remarkup field, added files to it, saw them attach to the task when changes were saved.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5916

Differential Revision: https://secure.phabricator.com/D10335