Move rendering to SearchEngine for Calendar

Summary:
Ref T4986. This one needs getApplicationURI() so make it a little beefier to deal with that.

(It would be vaguely nice to somehow share the handle and application stuff between Controllers and Engine classes like this, but I don't immediately see a clean way to do it without traits. Not a big deal, in any case.)

Test Plan:

• Viewed Calendar.
• Made a Calendar panel.
• Viewed feed.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9012