order clause refactoring, allow to use custom sql ordering strategies

[Fivell]

fixes #1590
fixes #3085

• added ability to use custom class for handling ordering per resource

    ActiveAdmin.register Post do
      config.order_clause = MyOrderClause
    end
  

• added dsl to customize ordering strategy per column

    ActiveAdmin.register User do
      order_by(:full_name) do |order_clause|
        ['COALESCE(NULLIF(last_name, ''), first_name), first_name', order_clause.order].join(' ')
      end
    end

[timoschilling]

Please add some line to the docs and a line into the changelog

[Fivell]

@timoschilling , take a look pls