Implement export

[mzur]

Add an "Export" page to the admin area. Implement the export of users, label trees and volumes as described here. See if everything can be stored in a single JSON for export of large volumes or if annotations should be stored in a streamable CSV along with the rest of the data as JSON in a ZIP archive.

• [x] User export
• [x] Label tree export
• [x] Volume export
• [x] Config to enable only specific exports

[mzur]

As the user export file contains the password hashes, add a warning to the docs to handle the file with care.

[mzur]

Forgot the image labels in the volume export.