Allow admins to customize /user-redirect/ behavior

/user-redirect/ is used to help link to a particular url
in the logged in user's authenticated notebook. For example,
if I'm logged in as user 'yuvipanda' and hit the URL
/hub/user-redirect/git-pull, it'll redirect me to
/user/yuvipanda/git-pull. This is extremely useful in
connecting hub links to notebook server extensions, such
as nbgitpuller.

Admins might want to customize how this redirection is done -
for example, redirect users to different running servers
based on the nbgitpuller repository they are linking from.
Adding a hook here helps accomplish that.

jupyterhub/app.py

@@ -76,7 +76,7 @@ from .oauth.provider import make_provider
 from ._data import DATA_FILES_PATH
 from .log import CoroutineLogFormatter, log_request
 from .proxy import Proxy, ConfigurableHTTPProxy
-from .traitlets import URLPrefix, Command, EntryPointType
+from .traitlets import URLPrefix, Command, EntryPointType, Callable
 from .utils import (
     maybe_future,
     url_path_join,
@@ -1258,6 +1258,21 @@ class JupyterHub(Application):
         """
     ).tag(config=True)
 
+    user_redirect_hook = Callable(
+        None,
+        allow_none=True,
+        help="""
+        Callable to affect behavior of /user-redirect/
+
+        A callable that receives two parameters - a handler and the path passed to
+        `/user-redirect/<path>`, and returns a URL to redirect the user to. This
+        can be used to customize how the `user-redirect` URL works.
+
+        To get the default behavior of /user-redirect/ leave this property unset
+        or return None from your callable.
+        """
+    ).tag(config=True)
+
     def init_handlers(self):
         h = []
         # load handlers from the authenticator
@@ -2144,6 +2159,7 @@ class JupyterHub(Application):
             trusted_alt_names=self.trusted_alt_names,
             shutdown_on_logout=self.shutdown_on_logout,
             eventlog=self.eventlog,
+            app=self
         )
         # allow configured settings to have priority
         settings.update(self.tornado_settings)

jupyterhub/handlers/base.py

@@ -140,6 +140,10 @@ class BaseHandler(RequestHandler):
     def hub(self):
         return self.settings['hub']
 
+    @property
+    def app(self):
+        return self.settings['app']
+
     @property
     def proxy(self):
         return self.settings['proxy']
@@ -1470,11 +1474,22 @@ class UserRedirectHandler(BaseHandler):
 
     If the user is not logged in, send to login URL, redirecting back here.
 
+    If c.JupyterHub.user_redirect_hook is set, the return value of that
+    callable is used to generate the redirect URL.
+
     .. versionadded:: 0.7
     """
 
     @web.authenticated
-    def get(self, path):
+    async def get(self, path):
+        # If hook is present to generate URL to redirect to, use that instead
+        # of the default. The configurer is responsible for making sure this
+        # URL is right. If None is returned by the hook, we do our normal
+        # processing
+        url = None
+        if self.app.user_redirect_hook:
+            url = await self.app.user_redirect_hook(self, path)
+        if url is None:
             user = self.current_user
             user_url = url_path_join(user.url, path)
             if self.request.query: