Avoid creating duplicate events by respecting the UID field

[dbarnett]

For events that contain a UID field, use Google Calendar's event import feature instead of insert to avoid creating duplicates.

Note that events with UID populated may still get processed as new/updated events if the SEQUENCE number is also incremented (https://www.kanzaki.com/docs/ical/sequence.html).

Fixes #492 & #583.