Preview generation filters in the repository

[tusmester]

We need to be able to configure whether the system should generate preview images in certain cases. The goal is to prevent starting unnecessary preview generation tasks.

Another use case is to switch off preview generation temporarily in a subtree while importing to prevent overloading the servers.

• in a subtree (multiple paths)
• ~for special content types (e.g. special files)~ can be set on CTD

Configuration: Folder content type, 3-state choice field (ON, OFF, Inherited)