* FIX: allowed_theme_ids should not be persisted in GlobalSettings
It was observed that the memoized value of `GlobalSetting.allowed_theme_ids` would be persisted across requests, which could lead to unpredictable/undesired behaviours in a multisite environment.
This change moves that logic out of GlobalSettings so that the returned theme IDs are correct for the current site.
Uses get_set_cache, which ultimately uses DistributedCache, which will take care of multisite issues for us.