fix(caldav): event links in shared calendar notifications

[st3iny]

• Resolves: Shared calendar: links from mail or notifications to events broken calendar#5293

Summary

The link in event notifications is invalid if a calendar is shared. An incoming shared calendar is belonging to sharee's principal with the custom name <calendar>_shared_by_<sharer>.

I implemented a hack to change the link in those cases to make the links in notifications work again.

How to reproduce the bug?

1. Have 2 users: admin and user
2. Install and enable the Calendar app.
3. Log in as user and enable all push notifications regarding calendars (see screenshot below).
4. Log in as admin.
5. Create a calendar and share it with user.
6. Create an event in the calendar.
7. Log in as user.
8. Open the notifications (on the top right) and click on the event link.

Here: Clicking the link redirects to Calendar and opens the event.
Master: Clicking the link redirects to Calendar and shows an empty content Event does not exist.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[miaulalala]

[st3iny]

/backport to stable29

[st3iny]

/backport to stable28

[st3iny]

/backport to stable27