Include ICS in notifications

[Semir1212]

It would be an advantage if users who are not regularly in the network could add the invitations directly to their personal calendar.

[yurabakhtin]

@luke- @Semir1212 Done in PR #436.

[dantefromhell]

Thanks a lot for implementing this feature, highly appreciated.

I updated my calendar module but I haven't seen ICS attachments attached to either the Instant Notifications (via email) nor the Daily Summary.

How is this feature supposed to work exactly?

[dantefromhell]

I updated my calendar module but I haven't seen ICS attachments attached to either the Instant Notifications (via email) nor the Daily Summary.

Nevermind, I answered my own question in #439.
Turns out the way I tested adding user is a special code path.