Webpublisher:change create flatten image into tri state

[kalisp]

Brief description

CollectColorCodedInstances allows publish only flattened subset of 'image' family.

Description

Previously it was possible to set in Settings to create flatten subset of 'image' family from published subsets. But separate layers marked by publishable colors were published also. This PR brings more granularity, now it is possible to:

• create flattened 'image' subset of all publishable layers + create 'image' subset per publishable layer
• create ONLY flattened 'image' subset of all publishable layers
• not create flattened 'image' subset at all, only separate subsets per marked layer.

Documentation (add "type: documentation" label)

feature_documentation

Testing notes:

1. set project_settings/photoshop/publish/CollectColorCodedInstances/flatten_subset_template

3. mark some top layers in PS to be published
4. check that only flatten subset was produced

[ynbot]

Task linked: OP-3713 Change 'Create flatten image' into tri-state