Export/ import categories

[lislis]

The buttons already exist as placeholders in the frontend.

• export
  Basically merge the contents of localstorage categories and category_items into one object, write it into a buffer and start a download as json.

• import (needs a fileupload)
  Read the file as json and separate the categories from category_items and save them to the database.
  It probably makes sense to delete all previous entries here and just import into a clean table so we don't have to deal with duplicates? there could be a warning saying "Hey, importing will remove everything that you currently have"