MDL-16660: Add calendar import.

Adds an extra import section alongside the existing export in the
calendar views. Allows the user to import from either a file or a
URL as a subscription, with an optional polling interval of hourly,
daily, weekly, monthly, or annually. This subscription may be added
to the user, group, global or course calendars. These subscriptions
are tracked in a separate database table, and an extra column added
to the events table to relate them to the subscription. The event
uuid field is also expanded to allow for the RFC-2445 UID property.
Subscriptions are listed on the calendar view page, and can be
added and removed, manually polled, and the polling interval
adjusted. Subscription events are updated on cron.