Force setting the SNIKKET_DOMAIN as HTTP Host when talking to prosody

This is required if the portal is talking to a Prosody with
multiple HTTP domains and it doesn’t have http_default_host set...
In a true snikket, this doesn’t happen, but we all know...

snikket_web/prosodyclient.py

@@ -144,6 +144,7 @@ class HTTPSessionManager:
     async def _create(self) -> aiohttp.ClientSession:
         return aiohttp.ClientSession(headers={
             "Accept": "application/json",
+            "Host": current_app.config["SNIKKET_DOMAIN"],
         })
 
     async def teardown(self, exc: typing.Optional[BaseException]) -> None:
@@ -204,6 +205,7 @@ class HTTPAuthSessionManager(HTTPSessionManager):
             headers={
                 "Authorization": "Bearer {}".format(token),
                 "Accept": "application/json",
+                "Host": current_app.config["SNIKKET_DOMAIN"],
             }
         )