Add option for Preview Link generate route to render in admin theme

I think we need a slightly better name for the setting, I'm also questioning whether we really need all the plumbing for a config screen with a single checkbox on it. It could just be a hidden (and documented) config setting.

We also need an update hook to set the value for existing sites.

I think it's a pretty essential feature, so the setting should definitely be exposed in the UI. The plumbing is just necessary because this module doesn't have a settings form yet, I'm sure more settings will be added at some point in the future.

I renamed the config option to settings_route_admin_theme and improved the description a little bit.

If we're going to go down this route then we need tests for the new settings page and coverage for submitting the form rebuilding the route (i.e load generate page, check not admin theme, submit form, check admin theme on generate route).

Also need update hook as per #13

I don't believe this needs to be optional / opt-in. I think it's clear it should be an admin route, next to 'View', 'Edit', 'Delete', 'Revisions', etc. I'm not sure there is any UX benefit to using the page theme and having it inconsistent with everything else.

It's also a duplicate of 🐛 The generate-preview-link page renders in the site's theme RTBC , which I know was created after. But might be easier to push the individual changes through rather than globbing them together.

Closing as a dupe of 🐛 The generate-preview-link page renders in the site's theme RTBC however, I will transfer credit for those who worked on this issue. Thanks!