Basic @mentions support

Summary:
Provides basic Remarkup support for @mentions. No application integration yet so
these aren't terribly useful until that happens.

Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-83d68e7af6085ae928df/

Reviewers: tomo, mroch, jsp
Commenters: tomo
CC: aran, tomo, epriestley
Differential Revision: 517

src/applications/differential/parser/markup/DifferentialMarkupEngineFactory.php

@@ -42,6 +42,7 @@ class DifferentialMarkupEngineFactory {
     $rules[] = new PhabricatorRemarkupRuleDiffusion();
     $rules[] = new PhabricatorRemarkupRuleManiphest();
     $rules[] = new PhabricatorRemarkupRuleImageMacro();
+    $rules[] = new PhabricatorRemarkupRuleMention();
 
     $custom_rule_classes =
       PhabricatorEnv::getEnvConfig('differential.custom-remarkup-rules');