R1:33a0731619e8

Give "RemarkupControl" support for tracking metadata

Summary: Ref T13603. This will support explicit handling of attached files.

Test Plan: Adjusted new input to have "text" input type, used it alongside additional upcoming changes, saw sensible metadata behavior.

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21827