Add option to override host in displayed url

This commit introduces a new alias `connect-host` to override the host
info displayed at launch with a custom string.

It is intended to be used when the app is run in an environment where
the url to display to the users is not detectable reliably (proxified or
containerized setups for example).

notebook/notebookapp.py

@@ -548,6 +548,7 @@ def start(self):
     'notebook-dir': 'NotebookApp.notebook_dir',
     'browser': 'NotebookApp.browser',
     'pylab': 'NotebookApp.pylab',
+    'connect-host': 'NotebookApp.connect_host',
 })
 
 #-----------------------------------------------------------------------------
@@ -666,6 +667,14 @@ def _valdate_ip(self, proposal):
             value = u''
         return value
 
+    connect_host = Unicode(u'', config=True,
+        help=_("""Custom host to show in displayed URL.
+
+        Replace actual host, including protocol, address, port and base URL,
+        with the given value when displaying URL to the users. Do not change
+        the actual connection URL.""")
+    )
+
     port = Integer(8888, config=True,
         help=_("The port the notebook server will listen on.")
     )
@@ -1339,11 +1348,14 @@ def init_webapp(self):
 
     @property
     def display_url(self):
-        if self.ip in ('', '0.0.0.0'):
-            ip = socket.gethostname()
+        if self.connect_host:
+            url = self.connect_host + '/'
         else:
-            ip = self.ip
-        url = self._url(ip)
+            if self.ip in ('', '0.0.0.0'):
+                ip = socket.gethostname()
+            else:
+                ip = self.ip
+            url = self._url(ip)
         if self.token:
             # Don't log full token if it came from config
             token = self.token if self._token_generated else '...'