Causes errors on pre-existing guides.

InvalidArgumentException: Field localgov_guides_list_layout is unknown.

You're going to need to either check the field exists before using it or add an update hook to install this into existing sites.

@msayoung @stephen-cox this PR is updated now to check for the field before acting on it.

Do we want to add this field to existing installs as well as new installs?

I will leave the decision on whether to add this to existing installs to someone else, but I not it's not straightforward to add is manually so might be good to push it out if it's useful.

Looks fine to me now, but before we can merge this we'll want all the tests passing.

Also just tested and working fine. Good question about existing sites, I agree that probably yes ...?

Discussing at Merge Tuesday - would a site wide setting for this be better?