Schema Editor: Allow persistence of sample data / upload of such sample data

- Specification of the format: text (incl. csv), xml, json? ->On schema-level, only XML is currently supported

(from redmine: issue id 330, created on 2018-01-22tgradl, closed on 2018-01-22)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information