Cleaning up, completing, bugfixing, adding features to iCalendar export of

Cleaning up, completing, bugfixing, adding features to iCalendar export of
Moodle calendar.

* Adding strings
* Adding more information to iCalendar output
* Making iCalendar output compatible with a wider range of calender software
* Removing (non-functional) advanced options for now, and other superfluous
code
* Paying attention to all / course events option
* Adding authentication token system to allow subscription
* Adding JavaScript to generate URL for subscription
* Adding links from calendar page

Author: Andrew Walbran <andrew.walbran@catalyst.net.nz>