New include for Google Calendar iframe

Pulls sources and display colors from _data/calendars.yml. src is a
Google Calendar calendar ID, which can be a GMail address for a user's
main calendar. Color (optional) is RGB hex beginning with a '#'.

Include parameters are height (required) and width (optional).
Add Google calendars

Include on front page.
Merge branch 'calendar-include'

More work to be done, but this is enough to publish for now.