Fix loading of sibling extensions

[afshin]

Prior to this change, an extension app either had to load all all extensions using load_other_extensions, or else only load itself. This change allows an extension app to declare a set of sibling extensions to load using serverapp_config.

[codecov-commenter]

Codecov Report

Merging #485 (0d11ffc) into master (8bfdd40) will increase coverage by 0.03%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #485      +/-   ##
==========================================
+ Coverage   77.69%   77.73%   +0.03%     
==========================================
  Files         106      106              
  Lines        9255     9269      +14     
  Branches      996      997       +1     
==========================================
+ Hits         7191     7205      +14     
  Misses       1707     1707              
  Partials      357      357              

Impacted Files	Coverage Δ
jupyter_server/extension/application.py	72.68% <83.33%> (+0.18%)	⬆️
...pyter_server/tests/extension/mockextensions/app.py	90.00% <100.00%> (+0.34%)	⬆️
jupyter_server/tests/extension/test_app.py	100.00% <100.00%> (ø)
jupyter_server/serverapp.py	67.12% <0.00%> (+0.10%)	⬆️

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