Add basic authentication in the server (#1670)

* Add basic authentication in the server

A pattern for creating authentication methods is introduced, with three
authentication types defined:
  none - no authentication
  session - like the current session based auth used for user annotations
  test - used to test the login/logout process end to end

The config endpoint now returns informations about the authentication, like if
the user is authenticated and their username.  The redirect uri's for login and
logout are also returned if the authentication type requires login

This is the first a several PRs for authentication.

*. Update server tests to avoid hardcoded ports

test_api and test_nan_rest now use a common function for starting a test server,
than will initially choose a random port.

server/common/default_config.py

@@ -14,6 +14,15 @@ server:
     server_timing_headers: false
     csp_directives: null
 
+  authentication:
+    # The authentication types may be "none" or "session"
+    # none:  No authentication support, features like user_annotations must not be enabled.
+    # session:  A session based userid is automatically generated.
+    type: session
+
+    # a dictionary of parameters that may be required for an authentication type
+    params: null
+
   multi_dataset:
     # If dataroot is set, then cellxgene may serve multiple datasets.  This parameter is not
     # compatible with single_dataset/datapath.
@@ -132,6 +141,9 @@ dataset:
     about_legal_tos: null
     about_legal_privacy: null
 
+    # allow authentication support
+    authentication_enable: true
+
   presentation:
     max_categories: 1000
     custom_colors: true