[FEATURE] Create BE user group based on template user group

[stigfaerch]

Make it possible to select a specific BE user group as template for creation of the new BE user group. Id's in the tsconfig field should be converted.

[stigfaerch]

This also makes it possible to have default subgroups attached to the site group

[stigfaerch]

This is also something that I implemented in
https://gitlab.com/dkm-extensions/site-generator-extended/