Steps to make thumbnail variant more "CMS-only"

We discussed that the imageset's thumbnail variant is currently represented as a default/example image variant of an image set. That's not its original intent. To make the thumbnail more of a CMS-internal thing, the following actions are considered:

• hard-code the dimensions and creation of the thumbnail, quit bootstrapping the corresponding configuration.
• stop presenting the thumbnail as one of the variants in the CMS gallery.

BC constraints need to be evaluated before implementing this. Target release is 10.