add an iCal feed for events

[jenlampton]

The Google Calendar is currently totally separate from backdropcms.org/news/events. Right now it's totally manual that someone (usually @jenlampton) copies important events into the Google Calendar. It would be great automatically generate an ICS feed to remove this manual, restricted action.

This is a follow-up to #890

Solution:

• add the https://backdropcms.org/project/viewsicalexport modue
• Create a new views display to generate the feed
• Update the backdrop events calendar to pull from this feed