Support iCalendar integration

[philipmjohnson]

Many students have requested the ability to "subscribe" to Schedule page events so that they would show up on their own Google Calendar without having to look at the Schedule page.

It would appear that this could be possible by updating the Morea plugin to write out an iCalendar calendar file containing the latest entries each time the system is built. The following Ruby library appears to support that:

https://github.com/icalendar/icalendar

Then we could just provide the URL to that file to interested students.

It appears that there are issues with the frequency with which Google syncs your calendar with these feeds. It could hours or days before it happens. This might make this feature less useful (or even unrecommended).