Use PhabricatorTagView in user mentions

Summary:
Refs T1048 - Use `PhabricatorTagView` in user mention remarkup rules. Emits hovercards now :D

Also introduces `PhabricatorTagView::COLOR_INEXISTENT`

Test Plan: Inspected HTML to find sigil & meta ref. Hovered above them (with hovercard patch, sold separately). Hovercard appeared.

Reviewers: epriestley, chad, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1048

Differential Revision: https://secure.phabricator.com/D5573