Fixes #8426 - Move scoped search into concerns

Scoped search definitions can be cumbersome at times and don't really
define the business logic of a model. I think hiding them under the
carpet by moving most of the definitions to concerns (as we do with
    hostext) we can later on include would keep the code cleaner.

There are 3 or 4 models with just 1 scoped_search attribute I didn't
bother to move. Later commits that move them into concerns when they
become bigger can just reference this issue.