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)