Special case ExtensionApp that starts the ServerApp

[afshin]

This significantly simplifies how ExtensionApps initialize a ServerApp. ServerApp.initialize now has an optional argument, starter_extension, that special-cases the extension which started the server and loads server-specific config from that extension.

As a nice side effect, the default_url and open_browser traits are handled more cleanly.

[codecov-io]

Codecov Report

Merging #401 (6f9a43a) into master (5013f62) will increase coverage by 1.06%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   68.87%   69.93%   +1.06%     
==========================================
  Files          57       57              
  Lines        6319     6313       -6     
  Branches      846      846              
==========================================
+ Hits         4352     4415      +63     
+ Misses       1678     1596      -82     
- Partials      289      302      +13     

Impacted Files	Coverage Δ
jupyter_server/serverapp.py	64.74% <69.23%> (+4.65%)	⬆️
jupyter_server/extension/application.py	72.30% <73.68%> (+11.30%)	⬆️
jupyter_server/extension/manager.py	86.74% <80.00%> (-0.55%)	⬇️
jupyter_server/utils.py	63.46% <100.00%> (ø)
jupyter_server/base/handlers.py	68.24% <0.00%> (+0.67%)	⬆️

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[blink1073]

Thanks!