Import improvements

[juliushaertl]

• [x] https://github.com/nextcloud/tables/issues/833
• [x] https://github.com/nextcloud/tables/issues/365
• [x] https://github.com/nextcloud/tables/issues/838
• [x] https://github.com/nextcloud/tables/issues/836
• [ ] https://github.com/nextcloud/tables/issues/837

Further ideas to be filed separately:

• [ ] Collect and list failed rows and columns instead of just counting
• [ ] The backend code could need some refactoring
• [ ] Skip empty rows if detected https://github.com/nextcloud/tables/pull/845#issuecomment-1969322148
• [ ] If we have failed rows collected, allow to manually adjust them
• [ ] Epic: We could dryrun the import and present the user with the expected outcome in table column format and a preview