Use RemarkupControl in Differential inline comment UI

Summary: Fixes T4317. Update the "inline comment" control to a RemarkupControl. This could maybe use some padding/spacing/design touches eventually but seems OK for the moment.

Test Plan: {F101825}

Reviewers: chad, btrahan

Reviewed By: chad

CC: chad, aran

Maniphest Tasks: T4317

Differential Revision: https://secure.phabricator.com/D7969