The Problem

Recurrence exceptions have weird behavior where a 'recurrence exception' can be treated
as the root of a recurrence sequence, by appending, for example, /2 to the uri.

Concrete example

https://phabricator.wikimedia.org/E196 is recurrence 2 of https://phabricator.wikimedia.org/E179
but if you visit https://phabricator.wikimedia.org/E196/3 you get a ghost event that appears to
be a recurrence of E196.

I think more consistent behavior would be to treat E196/3 as recurrence
3 of E179 which makes it possible to easily navigate through a series even when some of the
events are ghosts and others are recurrence exceptions, etc.

How I've addressed the problem

1. For recurring sequences, base the sequence start date on the original event instead of the "recurrence exception" instance.
2. Added simple navigation buttons for next/previous recurrence.

• View a recurring event,
• Navigate through a few recurrences in the sequence
• edit a ghost recurrence, see that it gets properly turned into a recurrence exception
• navigate past a recurrence exception, see that it's possible to navigate through the full series without breaking.

Change-Id: I4b8e1b9dfa92ab0ad0bcbbd844f4b21f965cf924