If saving in Expert mode and the frontmatter field contained invalid
yaml, the page content was changed to the error description.
Instead, avoid saving the page and return an error in the Admin
interface.
Add removing files from a blueprint to Admin, moved from Pro. Drop
`showuploaded`, `ispluginconfig`, `showuploadedpreview` options from
the File field. Added a "blueprint" option to specify the field name
and type, e.g. `plugins.admin` or `themes.antimatter`. Dropped the
multiple option, as it'd need more handling that now it's not there.
