Move user_for_paper_trail to ApplicationController

instead of putting it on ActionController::Base.
Putting it on ActionController::Base was overreaching and ended up causing exceptions for others controllers like spree_wombat handlers where try_spree_current_user wasn't defined.
If we want to enable papertrail on controllers that are not our own then we can just do the papertrail standard thing of defining "user_for_paper_trail" on those controllers (they'll likely need different logic anyway)