Add a limit to amount of previews that can be generated by a single call to pre-generate

[Torbins]

Feature request

Which Nextcloud Version are you currently using: - 29.0.6

Sometimes user can add a folder to list of synchronizations in his NextCloud app, and this folder would have thousands of photos. This will lead to a situation when previews for all new photos will be generated during next call to pre-generate, making system very slow to respond to other requests, and maybe will even interrupt folder synchronization itself. Please add a limit to amount of previews that can be generated by a single call to pre-generate. Maybe even two limits: one total amount of previews and another per-user amount. Another idea is to have a limit on total size of files that can be processed during single call of pre-generate.

With this feature implemented, it should be possible to use nextcloud background jobs instead of manual shell commands.