Use subclasses to expose types of servers

These changes break the existing ServerApp into 3 subclasses of
BaseServerApp - ServerApp (for serving notebooks), KernelServerApp
(for serving kernels) and ContentsServerApp (for serving content).

This approach allows for more specific behaviors in the various
subclasses, but, I suspect, breaks the ExtensionApp modeling, since
that appears to assume extensions are applied to ServerApp.  As a
result, more investigation is required if we wanted KernelServerApp
or ContentsServerApp to have the ability to be extended, rather than
only subclassed.

All tests currently pass against ServerApp, and KernelServerApp appears
to startup fine and only serve the application endpoints.

jupyter_server/extension/application.py

@@ -15,15 +15,15 @@
 
 from jupyter_core.application import JupyterApp
 
-from jupyter_server.serverapp import ServerApp, aliases, flags
+from jupyter_server.serverapp import ServerApp
 from jupyter_server.transutils import _
 from jupyter_server.utils import url_path_join
 from .handler import ExtensionHandler
 
 # Remove alias for nested classes in ServerApp.
 # Nested classes are not allowed in ExtensionApp.
 try:
-    aliases.pop('transport')
+    ServerApp.aliases.pop('transport')
 except KeyError:
     pass
 
@@ -124,7 +124,7 @@ def _prepare_templates(self):
 # Aliases and Flags
 #-----------------------------------------------------------------------------
 
-flags['no-browser']=(
+ServerApp.flags['no-browser']=(
     {'ExtensionApp' : {'open_browser' : True}},
     _("Prevent the opening of the default url in the browser.")
 )
@@ -181,8 +181,8 @@ def _validate_extension_name(self, proposal):
         ServerApp,
     ]
 
-    aliases = aliases
-    flags = flags
+    aliases = ServerApp.aliases
+    flags = ServerApp.flags
 
     subcommands = {}