allow handlers to work without an authorizer in the Tornado settings

[Zsailer]

Fixes #713.

We worked on this issue during contributing hour.

Allows the handlers in jupyter_server to be used without its tornado web app by setting the authorizer to None when it's not found in the tornado settings. Adding this to maintain backwards compatibility for now.

It also raises a warning to inform libraries like Voila that they should include an authorizer in the settings in future releases of Jupyter Server.

[Zsailer]

This PR raises the warning every time an authorizer isn't found. Ideally, this would only happen the first time it gets called, but we couldn't get this working within the contributing hour time block. I'll try to carve out time to finish this later today, unless anyone wants to pick up the task.

We will likely need this merged by Monday to unblock #716 (see #696 (comment))

[blink1073]

Kicking CI

[codecov-commenter]

Codecov Report

Merging #717 (1a51c0d) into main (426a4c6) will decrease coverage by 0.04%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##             main     #717      +/-   ##
==========================================
- Coverage   70.84%   70.79%   -0.05%     
==========================================
  Files          62       62              
  Lines        7490     7496       +6     
  Branches     1187     1188       +1     
==========================================
+ Hits         5306     5307       +1     
- Misses       1829     1833       +4     
- Partials      355      356       +1     

Impacted Files	Coverage Δ
jupyter_server/auth/decorator.py	78.94% <37.50%> (-11.38%)	⬇️
jupyter_server/base/handlers.py	65.72% <100.00%> (ø)
jupyter_server/serverapp.py	65.38% <0.00%> (-0.13%)	⬇️
jupyter_server/extension/application.py	74.66% <0.00%> (+0.34%)	⬆️

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[blink1073]

Thanks!

[Zsailer]

@blink1073 did you mean to close this without merging? I think we should merge and release this as soon as possible to handle #713.