Clean up the Clean Logic

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We need to improve the logic around cleaning. We should follow the lead of git clean and support flags for the jupyter lab clean command here.

The current logic only cleans the staging directory:

jupyterlab/jupyterlab/commands.py

Line 357 in e2fd4c8

for name in ['staging']:

in the app directory.

I think we should add flags for cleaning:

• The static directory
• The settings directory (for recovery - warn if there are local linked extensions)
• The extensions directory (for recovery)
• The entire app directory (for recovery)

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@telamonian, do you want to take on this one?