Show a trigger rule summary on the rule view page

Summary: Ref T5474. When you view the main page for a rule, show what the rule does before you actually edit it.

Test Plan:
Viewed a real trigger, then faked invalid/unknown rules:

{F6300211}

{F6300212}

{F6300213}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5474

Differential Revision: https://secure.phabricator.com/D20303