build: add code coverage (#2163)

* Update plugins.sbt

* build: add coverage, imports fixing

* chore: apply fixes

* chore: apply fixes

* chore: apply fixes

* chore: apply fixes

* remove private

* chore: tune down scalafix

* fix UserHandlerSpec warnings

* fix naming

* fix some get methods warnings in admin groups

* complier config changes

* review + formatting fixes

* add few more warn fixes

* revert non-satisfying changes

* update compiler config + some fixes

* silenced twirl templates warnings

* get rid of warnings

* undo two nowarns

Co-authored-by: irinaschubert <irina.schubert@dasch.swiss>
Co-authored-by: Marcin Procyk <marcin.procyk@dasch.swiss>