zerver: Migrate some files to typed_endpoint.

Migrates `invite.py`, `registration.py` and
`email_mirror.py` to use `typed_endpoint`.

zerver/views/email_mirror.py

@@ -3,16 +3,17 @@ from django.http import HttpRequest, HttpResponse
 from zerver.decorator import internal_api_view
 from zerver.lib.email_mirror import mirror_email_message
 from zerver.lib.exceptions import JsonableError
-from zerver.lib.request import REQ, has_request_variables
 from zerver.lib.response import json_success
+from zerver.lib.typed_endpoint import typed_endpoint
 
 
 @internal_api_view(False)
-@has_request_variables
+@typed_endpoint
 def email_mirror_message(
     request: HttpRequest,
-    rcpt_to: str = REQ(),
-    msg_base64: str = REQ(),
+    *,
+    rcpt_to: str,
+    msg_base64: str,
 ) -> HttpResponse:
     result = mirror_email_message(rcpt_to, msg_base64)
     if result["status"] == "error":