Update all places where config is need for read-only use to load as immutable instead of editable

Closes #3541464

I took care to check the code for places where it is in fact setting values and then saving them to avoid any breakage. However, I did notice a few places where it sets config values but never actually calls the save method (e.g. in the SpamMasterCleanUpService class), so it might be good to check those to see if they are working as expected. That would be a separate issue, though.