auth: Rewrite data model for tracking enabled auth backends.

So far, we've used the BitField .authentication_methods on Realm
for tracking which backends are enabled for an organization. This
however made it a pain to add new backends (requiring altering the
column and a migration - particularly troublesome if someone wanted to
create their own custom auth backend for their server).

Instead this will be tracked through the existence of the appropriate
rows in the RealmAuthenticationMethods table.

zerver/lib/server_initialization.py

@@ -7,11 +7,13 @@ from zerver.lib.user_groups import create_system_user_groups_for_realm
 from zerver.models import (
     Realm,
     RealmAuditLog,
+    RealmAuthenticationMethod,
     RealmUserDefault,
     UserProfile,
     get_client,
     get_system_bot,
 )
+from zproject.backends import all_implemented_backend_names
 
 
 def server_initialized() -> bool:
@@ -28,6 +30,14 @@ def create_internal_realm() -> None:
     RealmUserDefault.objects.create(realm=realm)
     create_system_user_groups_for_realm(realm)
 
+    # We create realms with all authentications methods enabled by default.
+    RealmAuthenticationMethod.objects.bulk_create(
+        [
+            RealmAuthenticationMethod(name=backend_name, realm=realm)
+            for backend_name in all_implemented_backend_names()
+        ]
+    )
+
     # Create some client objects for common requests.  Not required;
     # just ensures these get low IDs in production, and in development
     # avoids an extra database write for the first HTTP request in