Track which user performed an action
Track which user performed an action

This commit adds the current user as a header to gds-api-adapters. The
header will be passed down to publishing-api, which will save it in
it's history (events & actions).

https://trello.com/c/HQ28fhrF

alphagov/content-tagger,alphagov/content-tagger,alphagov/content-tagger