Do not index the image field as Text

Only index the image field as a Keyword based exlusively on the value of `$document->getImageUrl`.

Currently all mapped fields with a `role` are indexed both internally and "publically". But things like `imageUrl` should not be searchable. Also the MediaBundle replaces the `imageUrl` with the actual imageUrl instead of the mediacontainer, but it did not affect the original search mapping.
Index all fields which ARE NOT assigned a role.