Fix new power level event being deduplicated, thus not persisted

Turns out we were overridding the content of the old power level event in the room, which
means that when it came time to persist the new power level event, the code compared the
content of the two and determined that the new state event was a duplicated, and thus it
wasn't persisted - so clients didn't see it down sync, and the new power levels were not
enforced after a server restart.

Copying the old power levels dict instead of using it directly solves the issue.

synapse/third_party_rules/access_rules.py

@@ -473,7 +473,7 @@ async def _freeze_room_if_last_admin_is_leaving(
         )  # type: EventBase
         if not power_level_state_event:
             return
-        power_level_content = power_level_state_event.content
+        power_level_content = power_level_state_event.content.copy()
         if not isinstance(power_level_content, dict):
             # The power level content has been set to something other than a dict...
             # bail out.