Add DEFAULT_WORKSPACE

[ellisonbg]

This is the server side change that goes with this commit:

jupyterlab/jupyterlab@daa7afe

The idea is to replace the default workspace string 'default' with char code 0 to avoid conflicts with user defined workspaces.

[codecov-commenter]

Codecov Report

Merging #112 into master will decrease coverage by 0.04%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   60.43%   60.39%   -0.05%     
==========================================
  Files          19       19              
  Lines        1466     1467       +1     
==========================================
  Hits          886      886              
- Misses        580      581       +1     

Impacted Files	Coverage Δ
jupyterlab_server/handlers.py	53.27% <50.00%> (-0.26%)	⬇️

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[afshin]

Okay I think what is missing is modifying the workspaces_handler.py file as well. But this raises a question of what the REST endpoint URL for the default workspace needs to be. I think maybe going with this null character approach means we need a special URL specifically for the default workspace.

[ellisonbg]

Talked to @afshin today and we believe it is best to stick with the default string - closing this PR.