Add a simple symbol lookup interface for cross-references

Summary: This will get fancier, but here's a basic interface for doing symbol
lookups. Still all pretty tentative.

Test Plan: Looked up various things, got some sensible results.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: tuomaspelkonen

CC: aran, tuomaspelkonen

Differential Revision: 900

src/aphront/default/configuration/AphrontDefaultApplicationConfiguration.php

@@ -252,6 +252,7 @@ class AphrontDefaultApplicationConfiguration
           '$' => 'DiffusionCommitListController',
           '(?P<username>\w+)/$' => 'DiffusionCommitListController',
         ),
+        'symbol/(?P<name>[^/]+)/$' => 'DiffusionSymbolController',
       ),
 
       '/daemon/' => array(