Setting to disable redirect file browser launch

When it was originally added, the PR for launching a browser via a redirect file (jupyter#4260) created some issues for some environments-namely WSL, Chromebook, and Android (jupyter#4346 (comment), jupyter/help#496). The reason for the break in these environments is due to the file structure/path differences between the runtime and the browser. 

This commit adds a setting to the `jupyter_notebook_config.py` that allows users to disable the use of redirect file for launching a browser in favor of the original, yet visible, URL + token approach. This setting: `c.NotebookApp.use_redirect_file` will be set to True by default.

notebook/notebookapp.py

@@ -223,6 +223,7 @@ class NotebookWebApplication(web.Application):
             base_url=base_url,
             default_url=default_url,
             template_path=template_path,
+            use_redirect_file=jupyter_app.use_redirect_file,
             static_path=jupyter_app.static_file_path,
             static_custom_path=jupyter_app.static_custom_path,
             static_handler_class = FileFindHandler,
@@ -539,6 +540,20 @@ flags['allow-root']=(
     _("Allow the notebook to be run from root user.")
 )
 
+flags['use_redirect_file']=(
+    {'NotebookApp' : {'use_redirect_file' : True}},
+    """Disable launching browser by redirect file
+
+     For some environments (like Windows Subsystem for Linux (WSL) and Chromebooks),
+     launching a browser using a redirect file (introduced in notebook versions 
+     > 5.7.2) can lead the browser failing to load. This is because of the 
+     difference in file structures/paths between the runtime and the browser. 
+     
+     Disabling this setting will  by setting to False will disable this behavior, 
+     allowing the browser to launch by using a URL and visible token (as before).
+     """
+)
+
 # Add notebook manager flags
 flags.update(boolean_flag('script', 'FileContentsManager.save_script',
                'DEPRECATED, IGNORED',
@@ -645,6 +660,19 @@ class NotebookApp(JupyterApp):
         help=_("Whether to allow the user to run the notebook as root.")
     )
 
+    use_redirect_file = Bool(True, config=True,
+        help="""Disable launching browser by redirect file
+
+     For some environments (like Windows Subsystem for Linux (WSL) and Chromebooks),
+     launching a browser using a redirect file (introduced in notebook versions 
+     > 5.7.2) can lead the browser failing to load. This is because of the 
+     difference in file structures/paths between the runtime and the browser. 
+     
+     Disabling this setting will  by setting to False will disable this behavior, 
+     allowing the browser to launch by using a URL and visible token (as before).
+     """
+    )
+
     default_url = Unicode('/tree', config=True,
         help=_("The default URL to redirect to from `/`")
     )
@@ -1805,7 +1833,13 @@ class NotebookApp(JupyterApp):
         if not browser:
             return
 
-        if self.file_to_run:
+        if not self.use_redirect_file:
+            uri = self.default_url[len(self.base_url):]
+
+            if self.token:
+                uri = url_concat(uri, {'token': self.token})
+
+        elif self.file_to_run:
             if not os.path.exists(self.file_to_run):
                 self.log.critical(_("%s does not exist") % self.file_to_run)
                 self.exit(1)
@@ -1820,10 +1854,17 @@ class NotebookApp(JupyterApp):
         else:
             open_file = self.browser_open_file
 
-        b = lambda: browser.open(
-            urljoin('file:', pathname2url(open_file)),
-            new=self.webbrowser_open_new)
-        threading.Thread(target=b).start()
+        if not self.use_redirect_file:
+            if browser:
+                b = lambda: browser.open(url_path_join(self.connection_url, uri),
+                                        new=self.webbrowser_open_new)
+                threading.Thread(target=b).start()
+
+        else:
+            b = lambda: browser.open(
+                urljoin('file:', pathname2url(open_file)),
+                new=self.webbrowser_open_new)
+            threading.Thread(target=b).start()
 
     def start(self):
         """ Start the Notebook server app, after initialization