Make closed/disabled results in the remarkup autocomplete more visually clear

Summary:
Ref T13114. See PHI522. Although it looks like results are already ordered correctly, the override rendering isn't accommodating disabled results gracefully.

Give closed results a distinctive look (grey + strikethru) so it's clear when you're autocompleting @mention... into a disabled user.

Test Plan:

Maniphest Tasks: T13114

Differential Revision: https://secure.phabricator.com/D19272