Update policy on updating config and message site admin when not applied

[andybroomfield]

This seems to be the first major test of our update policy: https://github.com/localgovdrupal/localgov/wiki/Developers:-Updates#configuration

We need decide how to test for config changes. The YAML config files will have changed so it probably makes sense to hard code the old config in the update hook, but we could create a config dir for old YAML config files indexed by update hook number.

We also need to agree what and how to display a message to the user when the new config isn't applied. I suppose this will have to be done by returning the message from the update hook as it shouldn't result in an error. Would be nice to be able to display warning messages, but this doesn't look possible.

Finally, any code that depends on the new configuration will have to test this exists. It would be nice if there are tests for this so that future code changes that don't do this are caught early.

@andybroomfield If you want to update directories before any of this is done then you should be fine reverting the view after. I think we'll have to implement all of this at some point, but it's a bit of work, so may time a little while to get done.

Originally posted by @stephen-cox in https://github.com/localgovdrupal/localgov_directories/issues/280#issuecomment-1576443814