Full-stack: Export of a user's categories and cards

[wisskirchenj]

Provide a new menu item in the navigation bar to export all user data as Json.

• create new DataExport page in Vue, that provides a FileSelector component (choose from Vuetify), that allows to specify a file-path, where all user data, i.e. all categories including the cards, that this user has at least READ access for, are stored into.
• All data are stored in one file.
• The frontend only selects the file path and then calls a new "to be implemented" export endpoint, that needs no additional parameters to the file path (optionally - but better later, we can allow to save only selected categories)

So all export functionality will be implemented in the server / backend:

• an ExportController, that provides the endpoint for frontend calls
• an ExportService, that loops over all categories, where the user has at least READ authority and stores a Json array where each category entry ha the following three properties
  • the collection name (key)
  • the associated collection of Cards data as nested array of Json
  • the metadata documents of the CATEGORY collection, that store the name, description and access rights for this category.

[wisskirchenj]

closed with merge of #89