Allow selecting mapping for thumbnail

[fm3]

The thumbnail.jpg route of the datastore should support selecting mappings, and the wk-side routes should also use that (let’s discuss how the user should specify what should be used there)

[fm3]

may be superseded by #3937

[fm3]

Note that as of #7190 the thumbnail route takes a mappingName parameter. The front-end does not yet make use of that, though. Not sure how the frontend would know which to choose.

[fm3]

mapping name should probably become a part of the view configuration #7412 then we could take it from there.

[fm3]

The mappingName is now part of the viewConfiguration, as of #7858
It should be passed to the thumbnail route called in ThumbnailService