Import: Add option to not delete empty folders

[zhzy0077]

• What does it implement / fix / improve? Why?
  For Synology Cloud Sync, it fails to sync to the WebDav when the folder doesn't exist. It won't create folders as part of the incremental sync. As a result, people see

2024-03-07 21:16:00 ERRO webdav: 'open /photoprism/import/x.jpg: no such file or directory' in PUT /import/x.jpg

The changes in this PR allow user to set a configuration to stop deleting empty folders after import.

• Are the changes related to an existing issue?
  No.

Acceptance Criteria:

• Features and enhancements must be fully implemented so that they can be released at any time without additional work
• Automated unit and/or acceptance tests are mandatory to ensure the changes work as expected and to reduce repetitive manual work
• Frontend components must be responsive to work and look properly on phones, tablets, and desktop computers; you must have tested them on all major browsers and different devices
• Documentation and translation updates should be provided if needed
• In case you submit database-related changes, they must be tested and compatible with SQLite 3 and MariaDB 10.5.12+

[CLAassistant]

All committers have signed the CLA.

[graciousgrey]

Thank you very much for your contribution. We will review it as soon as our time allows.

[lastzero]

@zhzy0077 Thanks again for your contribution and feedback! We're sorry we couldn't get to this sooner. 😞

The above commit will preserve any ".keep" and/or ".gitkeep" files in import directories. So if you add one, this will prevent the folder from being deleted, since only empty directories are deleted when using the move option.

Does this meet your needs? Any help in testing this with our preview build will be much appreciated:

• https://docs.photoprism.app/release-notes/#development-preview
• https://docs.photoprism.app/getting-started/updates/#development-preview

Since this is now implemented and the changes in the PR are no longer needed, I will close it and set the status to Preview for final testing.