Add initial skeleton outline of backdrop_users (or dataset_users?)

- Adds a many2many relation with data_sets and an email field
- Also adds very basic view of users in the admin app, but that doesn't work yet
Merge branch 'omg_the_backdrop_users_are_coming' of github.com:alphagov/stagecraft into omg_the_backdrop_users_are_coming

* 'omg_the_backdrop_users_are_coming' of github.com:alphagov/stagecraft:
  Working admin and test for unique email
  Add initial skeleton outline of backdrop_users (or dataset_users?)
  fix typo
  Improve the Bearer token help text
  Added links to Backdrop to data-set list page

Conflicts:
	stagecraft/apps/datasets/admin/backdrop_user.py
	stagecraft/apps/datasets/models/backdrop_user.py
wowee zowee it's a check_disabled decorator!
email not name