Load task from server if not found locally

[raimund-schluessler]

This implements #606. When a requested task is not found locally, we try to load it from the server.

Todo:

• We should also try to load the ancestor tasks (if any) of the requested task, so we can properly show the subtasks tree
• If we are in a collection view (e.g. /nextcloud/index.php/apps/tasks/#/collections/all/tasks/Nextcloud-lrr0rvhdcoqopte5k43n6.ics), we don't know which calendar a task belongs to, so we have to query all calendars I guess. Or is there a better way to find a task only knowing the last part of its uri @georgehrke @skjnldsv ?
• Don't loose subtask relations for tasks alread present in store when loading tasks by Uri or UID

What I don't like so much is that I have to watch the calendar (https://github.com/nextcloud/tasks/pull/608/files#diff-128fe9ee0ed5aeabe3c64281c67285c0R584), but it was the easiest way to load a task only after the calendars are loaded.

[codecov]

Codecov Report

Merging #608 into master will decrease coverage by 0.16%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #608      +/-   ##
=========================================
- Coverage    5.82%   5.66%   -0.17%     
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  Files          45      45              
  Lines        1939    1996      +57     
  Branches      343     351       +8     
=========================================
  Hits          113     113              
- Misses       1679    1730      +51     
- Partials      147     153       +6

[raimund-schluessler]

This should be good to go now.

[skjnldsv]

See comments :)

[raimund-schluessler]

See comments :)

Fixed it. Looks much better now with async/await.

[raimund-schluessler]

Rebased.