feat: do not delete folders flag

[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.