auth: Add ExternalAuthResult to manage data in authentication flows.

This new type eliminates a bunch of messy code that previously
involved passing around long lists of mixed positional keyword and
arguments, instead using a consistent data object for communicating
about the state of an external authentication (constructed in
backends.py).

The result is a significantly more readable interface between
zproject/backends.py and zerver/views/auth.py, though likely more
could be done.

This has the side effect of renaming fields for internally passed
structures from name->full_name, next->redirect_to; this results in
most of the test codebase changes.

Modified by tabbott to add comments and collaboratively rewrite the
initialization logic.

zerver/lib/test_helpers.py

@@ -23,7 +23,6 @@ from zerver.tornado import event_queue
 from zerver.tornado.handlers import allocate_handler_id
 from zerver.worker import queue_processors
 from zerver.lib.integrations import WEBHOOK_INTEGRATIONS
-from zerver.views.auth import get_login_data
 
 from zerver.models import (
     get_realm,
@@ -36,6 +35,8 @@ from zerver.models import (
     UserProfile,
 )
 
+from zproject.backends import ExternalAuthResult, ExternalAuthDataDict
+
 if TYPE_CHECKING:
     # Avoid an import cycle; we only need these for type annotations.
     from zerver.lib.test_classes import ZulipTestCase, MigrationsTestCase
@@ -445,10 +446,10 @@ def write_instrumentation_reports(full_suite: bool, include_webhooks: bool) -> N
                 print("   %s" % (untested_pattern,))
             sys.exit(1)
 
-def load_subdomain_token(response: HttpResponse) -> Dict[str, Any]:
+def load_subdomain_token(response: HttpResponse) -> ExternalAuthDataDict:
     assert isinstance(response, HttpResponseRedirect)
     token = response.url.rsplit('/', 1)[1]
-    data = get_login_data(token, should_delete=False)
+    data = ExternalAuthResult(login_token=token, delete_stored_data=False).data_dict
     assert data is not None
     return data