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.

In order for this to work we need to create an API client user on signonotron:

```
rake api_clients:create[name,email,application_name,permission]
```

See full task description here https://github.com/alphagov/signonotron2/blob/master/lib/tasks/api_clients.rake#L2-L15. Permission is just `signin`.

Then `her.rb` in alphagov-deployment will have to be modified to reflect these changes and `ENV['BEARER_TOKEN']` replaced with token that Signonotron will generate after task run.

Requires merge on tariff backend's pull request https://github.com/alphagov/trade-tariff-backend/pull/116.