Event recurrence registration end dates

Motivation

When creating recurring events the registration end date is the same for each event. For events that may be weeks, or months after the original event, this doesn't allow a user to register closer to the actual event date. They are forced to register within the registration period of the original event.

Proposed resolution

This patch modifies the registration end date for each recurrence of the original event. It does this by getting the timeframe between the original event registration period end value and the original event start value, and then changing each recurrence registration end date to reflect this same timeframe.

For example:
An event registration period ends on Dec. 1, and the event itself does not start until Dec. 5. There is a 4-day gap between when registration ends, and the event begins. Previously, each recurring event will end registration on Dec 1 like the original. Now, the registration end date will be modified to reflect the 4-day gap. So if a recurring event happens on Jan. 15, registration will end on Jan 11 instead of Dec. 1.