Treat cancelled tasks as completed

[raimund-schluessler]

With this PR we treat cancelled tasks as completed, so they don't show up as still to do. This resolves #1060.

This PR supersedes #1418.

Todo:

• Indicate canelled tasks (we show a cancelled icon in the checkbox)
• Also cancel not completed sub-tasks when cancelling a task
• Treat cancelled sub-tasks as completed in task list
• Add test functions for new behaviour

[codecov]

Codecov Report

Merging #1553 (6adab23) into master (1a0090a) will increase coverage by 0.01%.
The diff coverage is 41.66%.

@@            Coverage Diff             @@
##           master    #1553      +/-   ##
==========================================
+ Coverage   32.32%   32.33%   +0.01%     
==========================================
  Files          55       55              
  Lines        2803     2814      +11     
  Branches      528      532       +4     
==========================================
+ Hits          906      910       +4     
- Misses       1758     1762       +4     
- Partials      139      142       +3     

[raimund-schluessler]

@dargmuesli I packed a release, so you can test the changes. Please test it well and make sure everything makes sense and is consistent.

tasks.tar.gz

[dargmuesli]

I manually installed the .tar.gz file and reloaded the page. Now cancelled tasks aren't shown in All and Current lists, but under Completed. I've created a test task and a subtask under it. After cancelling the parent tasks, both tasks show up under Completed. Nice!