Opt in to config updates

[willguv]

Following on from localgovdrupal/localgov_geo#113 where there was a Merge Tuesday discussion about updating council config automatically...

@rupertj suggested giving councils the option to receive config updates from us. We think there are councils with few devs that will want everything, and some with devs that will want to control this themselves.

@finnlewis mentioned that we will need to define what we mean by config and other changes

[andybroomfield]

Just an FYI: BHCC uses Config sync and Config distro to do selective config imports.