Share files to cards polishing

[juliusknorr]

Follow up polishing from #2638

• [x] Upload on dragging #2701
• [ ] Add menu entry to either delete or unshare the file
• [ ] Configuation for target directory
• [ ] Propertly handle existing attachments
  • [ ] Add migration background job
  • [ ] Drop with occ command to migrate in a later version
• [ ] Conflict handling doesn't work with sharing since a file with the same name might be already present in the deck folder
• [ ] Upload could use chunking through webdav, but requires more work (probably something for a follow up)
• [ ] API documentation