Workbench should warn user if markup exists in a field value but the target field is not configured to render markup

[mjordan]

If the field formatter is available in the field's config (and it should be), Workbench can warn users during --check that values containing markup won't be rendered as expected.

[mjordan]

@seth-shaw-unlv suggests adding a config option to allow users to specify which format value (e.g. ‘basic_html’ or ‘full_html’) that field should use. This would let us provide more informative messages to users, do better markup validation, etc.