Clean up recurring event information on Calendar events

Summary: Ref T11326. This adds prev/next links for recurring events (ala D16179) and moves the "accept/decline" buttons closer to the invite list. This might need some fiddling, but should be a little more human-friendly.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16339