adds field for guide navigation layout type

[markconroy]

Closes #123

What does this change?

• Adds a new field called "List Layout" so editors can choose on a per-guide basis whether they want horizonal/vertical layout.
• Defaults to "Vertical (Single Column)"

How to test

Install Guides module
Create a guide overview and some guide pages
In Guide Overview, choose "Horizontal" or "Vertical" for the "List Layout"
Check that the Guide Nav responds accordingly NOTE it will need this PR from LocalGov base which has the required CSS.

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Thanks to Big Blue Door for sponsoring my time to work on this.

[msayoung]

Causes errors on pre-existing guides.

InvalidArgumentException: Field localgov_guides_list_layout is unknown.

[stephen-cox]

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.

[markconroy]

@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?

[stephen-cox]

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.

[msayoung]

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

[stephen-cox]

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