Add preview field / template support

[julrich]

This should work off of a new x-cms annotation, like x-cms-preview... typed to a simple string.
Should also work both for preview fields and templates (e.g. field:headline or template: [..] to differentiate).

Needed to have good previews for objects in repeatable arrays (e.g. components in a Section, or navItems in a Header).

[julrich]

Preliminary support was already added here:

• kickstartDS/schema@610de66
• kickstartDS/schema@4a12f96

Follow-up here: kickstartDS/schema#28