Handle mathjax_url properly

[afshin]

Fixes jupyterlab/jupyterlab#9605. The static/components url from the notebook server's mathjax_url config was getting prefixed with /lab/tree when we had a document open.

[codecov-io]

Codecov Report

Merging #150 (be0c253) into master (192c753) will decrease coverage by 0.05%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage   74.92%   74.86%   -0.06%     
==========================================
  Files          21       21              
  Lines        1675     1679       +4     
==========================================
+ Hits         1255     1257       +2     
- Misses        420      422       +2     

Impacted Files	Coverage Δ
jupyterlab_server/handlers.py	88.07% <60.00%> (-1.04%)	⬇️

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[blink1073]

https://pypi.org/project/jupyterlab-server/2.1.2/