Prevent editing not public tasks in lists shared with me

[raimund-schluessler]

This PR fixes #467 and implements #819 and #821.

Editing tasks in a calendar shared with me which have an access class other than PUBLIC is not possible anymore. Also changing the access class for tasks in a calendar shared with me will be prevented.

However, this only fixes the problem on the client side. Editing such tasks still needs to be prevented on the server side as well, see nextcloud/server#5551.

As a side effect, since we have to load the current user principal for this, we now show user avatars for the owner of a calendar shared with me in the app-navigation list and allow to unshare a calendar.

@rkaraba you might want to have a look as well.

[codecov]

Codecov Report

Merging #862 into master will decrease coverage by 0.96%.
The diff coverage is 9.09%.

@@            Coverage Diff             @@
##           master     #862      +/-   ##
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- Coverage   31.92%   30.95%   -0.97%     
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  Files          23       25       +2     
  Lines        1253     1305      +52     
  Branches      223      235      +12     
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+ Hits          400      404       +4     
- Misses        729      766      +37     
- Partials      124      135      +11

[raimund-schluessler]

@georgehrke How should we handle deleting non-public tasks/events in calendars shared with me? Do you plan to allow or forbid this in server? In this PR I treat such tasks as read-only, so deleting is not possible. But I would like to implement it the same way as in server.

[raimund-schluessler]

@georgehrke Ping.

[georgehrke]

@raimund-schluessler Yes, my plan for the server is to forbid editing / deleting.