Refactor / rebuild sharing

[brantje]

Due an unannounced API change in the Nextcloud core, i was forced to quickly copying their old code.
This resulted in a big, ugly file

This issue is about refactoring the sharing part in the app.
It will fix a lot of current sharing issues and add some new features.

• Switch to GUIDs in routes
• Update API docs to use GUID's
• Make a table that holds the shares
• Use binary operators for permissions
• Implement user to user sharing
  • Re-share permission
  • Edit permission
  • Delete permission
• Implement notebook sharing
  • Re-share permission
  • Edit permission
  • Delete permission
• Implement link sharing
  • Edit permission (include a warning that everyone with the link can edit)