Make `destroy` redirect to agents list

Before this, the `destroy` action was attempting to redirect the user back to the `show` action of the controller, which resulted in an error.

Fixes #531
On my system, `params[:return]` is equal to "%2Fscenarios%2F1" and
`scenarios_path` is equal to "/scenarios" so the regex won't match. When I
`html_safe`'d the `scenarios_path`, the line
    elsif params[:return] =~ /\A#{Regexp::escape scenarios_path}\/\d+\Z/
evals `nil`, indicating no match, when I delete an Agent from Scenario#show.