[MRG] link server extension subcommands to main app (#157)

* link server extension subcommands to main app

* Refactor extension subapp interface and add tests

* remove f-strings for 3.5 🤦

* change ModuleNotFoundError to ImportError for py3.5

CONTRIBUTING.rst

@@ -69,7 +69,7 @@ To run the Python tests, use::
 
 If you want coverage statistics as well, you can run::
 
-    py.test --cov notebook -v --pyargs jupyter_server
+    py.test --cov notebook -v
 
 Building the Documentation
 --------------------------
@@ -82,7 +82,7 @@ containing all the necessary packages (except pandoc), use::
 
     conda env create -f docs/environment.yml
     source activate server_docs  # Linux and OS X
-    activate notebook_docs         # Windows
+    activate server_docs         # Windows
 
 .. _conda environment:
     https://conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#creating-an-environment-from-an-environment-yml-file

jupyter_server/extension/__init__.py

@@ -1,2 +0,0 @@
-from .application import ExtensionApp
-from .handler import ExtensionHandler

jupyter_server/extension/application.py

@@ -418,7 +418,6 @@ def load_jupyter_server_extension(cls, serverapp, argv=[], **kwargs):
         extension.initialize(serverapp, argv=argv)
         return extension
 
-
     @classmethod
     def launch_instance(cls, argv=None, **kwargs):
         """Launch the extension like an application. Initializes+configs a stock server