Move open_browser to ProcessApp class

[jasongrout]

This follows on the changes in ExtensionApp to move open_browser to the top-level config.

[codecov-io]

Codecov Report

Merging #149 (d6c916f) into master (1f3113d) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   74.92%   74.92%           
=======================================
  Files          21       21           
  Lines        1675     1675           
=======================================
  Hits         1255     1255           
  Misses        420      420           

Impacted Files	Coverage Δ
jupyterlab_server/process_app.py	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1f3113d...d6c916f. Read the comment docs.

[blink1073]

Thanks!