Centralize app cleanup

[blink1073]

Potentially fixes #790

• Automatically clean up all server apps created in tests
• Add more cleanup logic to serverapp itself

Depends on jupyter/jupyter_client#772

TODO:

• Make sure terminals are shutting down - yes, as part of stop_extension
• Remove usages of jp_cleanup_subprocesses
• Release jupyter_client with Fix shutdown and cleanup behavior jupyter/jupyter_client#772

[blink1073]

Should hopefully pass now that jupyter_client 7.3 is released.

[codecov-commenter]

Codecov Report

Merging #792 (446dd4a) into main (73a03a6) will increase coverage by 0.02%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main     #792      +/-   ##
==========================================
+ Coverage   69.96%   69.99%   +0.02%     
==========================================
  Files          62       62              
  Lines        7368     7355      -13     
  Branches     1223     1225       +2     
==========================================
- Hits         5155     5148       -7     
+ Misses       1841     1835       -6     
  Partials      372      372              

Impacted Files	Coverage Δ
jupyter_server/services/kernels/kernelmanager.py	78.64% <76.19%> (-1.03%)	⬇️
jupyter_server/pytest_plugin.py	87.61% <83.33%> (+3.80%)	⬆️
jupyter_server/serverapp.py	64.96% <87.50%> (-0.15%)	⬇️
jupyter_server/utils.py	62.37% <0.00%> (+0.99%)	⬆️

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[Zsailer]

Wow, this is awesome. So much cleaner than before. Thank you, @blink1073!

[bollwyvl]

Great stuff.

[Zsailer]

@meeseeksmachine please backport to 1.x.

[blink1073]

@meeseeksdev please backport to 1.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 1.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 bf40316fcacc7cdd9c0c75534ae1b4ea78c26c9f

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #792: Centralize app cleanup'

4. Push to a named branch:

git push YOURFORK 1.x:auto-backport-of-pr-792-on-1.x

5. Create a PR against branch 1.x, I would have named this PR:

"Backport PR #792 on branch 1.x (Centralize app cleanup)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.