Use bearer token strategy for cross app communication

This change is for https://www.pivotaltracker.com/story/show/58791864

And should solve issues https://github.com/alphagov/trade-tariff-admin/issues/6 and https://github.com/alphagov/trade-tariff-admin/issues/5.


See more here https://github.com/alphagov/trade-tariff-admin/pull/8