add "Login with..." button

for custom authenticators that use external services (e.g. OAuth)

jupyterhub/auth.py

@@ -30,7 +30,18 @@ class Authenticator(LoggingConfigurable):
         If empty, allow any user to attempt login.
         """
     )
-    custom_html = Unicode('')
+    custom_html = Unicode('',
+        help="""HTML login form for custom handlers.
+        Override in form-based custom authenticators
+        that don't use username+password,
+        or need custom branding.
+        """
+    )
+    login_service = Unicode('',
+        help="""Name of the login service for external
+        login services (e.g. 'GitHub').
+        """
+    )
     
     @gen.coroutine
     def authenticate(self, handler, data):

jupyterhub/handlers/base.py

@@ -309,6 +309,7 @@ class BaseHandler(RequestHandler):
             prefix=self.base_url,
             user=user,
             login_url=self.settings['login_url'],
+            login_service=self.authenticator.login_service,
             logout_url=self.settings['logout_url'],
             static_url=self.static_url,
             version_hash=self.version_hash,

share/jupyter/hub/static/less/login.less

@@ -2,6 +2,13 @@
     display: table;
     height: 80vh;
     
+    .service-login {
+      text-align: center;
+      display: table-cell;
+      vertical-align: middle;
+      margin: auto auto 20% auto;
+    }
+    
     form {
         display: table-cell;
         vertical-align: middle;

share/jupyter/hub/templates/login.html

@@ -9,6 +9,12 @@
 <div id="login-main" class="container">
 {% if custom_html %}
 {{ custom_html }}
+{% elif login_service %}
+<div class="service-login">
+  <a class='btn btn-jupyter btn-lg' href='{{login_url}}'>
+    Sign in with {{login_service}}
+  </a>
+</div>
 {% else %}
 <form action="{{login_url}}?next={{next}}" method="post" role="form">
   <div class="auth-form-header">
@@ -51,7 +57,6 @@
   </div>
 </form>
 {% endif %}
-{% endif %}
 </div>
 {% endblock login %}