Accept or decline calendar shares through notifications

[tcitworld]

Notifications should be triggered by updateShares and actions should happen through ocs routes (so that it may be possible to accept shares directly from NC file clients).

Requires adding a field in oc_dav_shares to handle accepted status.

There should be a default_accept setting similar to the one in the file_sharing app to skip approval, or we can just reuse it.

Note: There's also the topic of handling sharing directly though CalDAV that's related to this (see https://tools.ietf.org/html/draft-pot-caldav-sharing-01, https://github.com/apple/ccs-calendarserver/blob/master/doc/Extensions/caldav-sharing.txt and https://github.com/nextcloud/server/issues/5050) but that's another topic.

[georgehrke]

We have to make sure that clicking the notification enables the calendar in the calendar-app. (Either display only that calendar or display previously visible calendars plus this calendar)

[tcitworld]

This means we need to have special intents from the notification ?