Refactor TopbarSweeper to remove rails-observer dependency
The rails-observer project is currently incompatible with Rails 5 and higher, and looks unlikely to be made compatible and released in the near future.
The TopbarSweeper uses rails-observer to clear user topbar caches when user/group/role etc. objects are updated from a controller, but this could be refactored into a model concern. The audited gem is also dropping its dependency, so refactoring TopbarSweeper would allow the dependency to be entirely dropped from Foreman (the original intention of #14434).
fixes #19035 - rewrite TopbarSweeper without rails-observers
Moves from the observer object into two mixins, one on the model and one
on the top-level controllers to observe creates/updates/destroys on
monitored models. Replaces rails-observers as it lacks Rails 5 support.