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/test/__init__.py

@@ -86,15 +86,14 @@ def random_string(n):
     return "".join(random.choice(string.ascii_letters) for _ in range(n))
 
 
-@contextmanager
-def test_server(command_line_args=[], app_config=None):
-    """A context to run the cellxgene server.
+def start_test_server(command_line_args=[], app_config=None):
+    """
     Command line arguments can be passed in, as well as an app_config.
     This function is meant to be used like this, for example:
 
     with test_server(...) as server:
-       r = requests.get(f"{server}/...")
-       // check r
+        r = requests.get(f"{server}/...")
+        // check r
 
     where the server can be accessed within the context, and is terminated when
     the context is exited.
@@ -104,7 +103,8 @@ def test_server(command_line_args=[], app_config=None):
     yaml config file, which this server will read and parse.
     """
 
-    port = int(os.environ.get("CXG_SERVER_PORT", DEFAULT_SERVER_PORT))
+    start = random.randint(DEFAULT_SERVER_PORT, 2**16 - 1)
+    port = int(os.environ.get("CXG_SERVER_PORT", start))
     port = find_available_port("localhost", port)
     command = ["cellxgene", "--no-upgrade-check", "launch", "--verbose", "--port=%d" % port] + command_line_args
 
@@ -128,10 +128,25 @@ def test_server(command_line_args=[], app_config=None):
     if tempdir:
         tempdir.cleanup()
 
+    return ps, server
+
+
+def stop_test_server(ps):
+    try:
+        ps.terminate()
+    except ProcessLookupError:
+        pass
+
+
+@contextmanager
+def test_server(command_line_args=[], app_config=None):
+    """A context to run the cellxgene server."""
+
+    ps, server = start_test_server(command_line_args, app_config)
     try:
         yield server
     finally:
         try:
-            ps.terminate()
+            stop_test_server(ps)
         except ProcessLookupError:
             pass