R1:4139305f73f0

Mark `renderActionDescription` as abstract

Summary:
I got caught by this... my custom Herald action doesn't implement this method and now I am hitting this exception:

```
Call to undefined method HeraldHipChatNotificationAction::renderActionDescription()

/usr/src/phabricator/src/applications/herald/adapter/HeraldAdapter.php:896
```

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.