Implement a username picker for synapse (#8942)

The final part (for now) of my work to implement a username picker in synapse itself. The idea is that we allow
`UsernameMappingProvider`s to return `localpart=None`, in which case, rather than redirecting the browser
back to the client, we redirect to a username-picker resource, which allows the user to enter a username.
We *then* complete the SSO flow (including doing the client permission checks).

The static resources for the username picker itself (in 
https://github.com/matrix-org/synapse/tree/rav/username_picker/synapse/res/username_picker)
are essentially lifted wholesale from
https://github.com/matrix-org/matrix-synapse-saml-mozilla/tree/master/matrix_synapse_saml_mozilla/res. 
As the comment says, we might want to think about making them customisable, but that can be a follow-up. 

Fixes #8876.

1 files changed, 7 insertions, 1 deletions

diff --git a/tests/unittest.py b/tests/unittest.py

index 39e5e7b85c..af7f752c5a 100644
--- a/tests/unittest.py
+++ b/tests/unittest.py
@@ -20,7 +20,7 @@ import hmac
 import inspect
 import logging
 import time
-from typing import Dict, Optional, Type, TypeVar, Union
+from typing import Dict, Iterable, Optional, Tuple, Type, TypeVar, Union
 
 from mock import Mock, patch
 
@@ -383,6 +383,9 @@ class HomeserverTestCase(TestCase):
         federation_auth_origin: str = None,
         content_is_form: bool = False,
         await_result: bool = True,
+        custom_headers: Optional[
+            Iterable[Tuple[Union[bytes, str], Union[bytes, str]]]
+        ] = None,
     ) -> FakeChannel:
         """
         Create a SynapseRequest at the path using the method and containing the
@@ -405,6 +408,8 @@ class HomeserverTestCase(TestCase):
                  true (the default), will pump the test reactor until the the renderer
                  tells the channel the request is finished.
 
+            custom_headers: (name, value) pairs to add as request headers
+
         Returns:
             The FakeChannel object which stores the result of the request.
         """
@@ -420,6 +425,7 @@ class HomeserverTestCase(TestCase):
             federation_auth_origin,
             content_is_form,
             await_result,
+            custom_headers,
         )
 
     def setup_test_homeserver(self, *args, **kwargs):