When mapping fields - add better handling for disabled or unpublished items.

Disabled items appear in the list of options for mapping database fields to Section fields, but then don’t work correctly.

We should either allow this properly for if you are building a page in advance of a release (with a secret URL) or properly show these items as disabled in the dropdown and don’t allow them to be selected. (Most likely the 2nd one is most consistent - when draft mode is built, the first option could be allowed for draft mode.)