Consider replacing concerns with dedicated classes & composition

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I read https://blog.codeship.com/when-to-be-concerned-about-concerns/ this morning and there are some pretty good points about:

• Using composition
• Limiting interface

We should start with a POC to show the advantages and how we could apply this broadly instead of moving stuff to concerns, in most cases just because some of the code is reused or because the main class becomes too big.

/cc @rspeicher @grzesiek :)

See also https://docs.gitlab.com/ee/development/software_design.html#distinguish-domain-code-from-generic-code