(feature request) Allow selecting document types for metadata

I had this issue when creating a new metadata type I wanted to apply to multiple document types. The only way I found to do that is to go to document types, press required or optional metadata and then add the new metadata type individually.

I think it would improve user experience considerably if a button "document types" was available on metadata/setup/type/list/. This could be implemented as a list of all document types with three checkboxes "n/a", "required", and "optional" for each document type. I think this would also provide a solution to #337 (closed).