Export event calendar to icalendar format

This change is branched off of `dnephin:some_build_things` so the diff includes those changes.

This branch adds an export for courses to icalendar format.  It uses  `webcal://` scheme so things like google calendar should know to update the calendar with new events as they get added.

Right now the link to calendar is unstyled, I could use some help with the style for the link and maybe even placement.

I've only added it to `branch_default`, but I noticed there was also a `branch_base`. What is the difference between these? Does it need to be added to both?