Added links to Backdrop to data-set list page

https://www.pivotaltracker.com/story/show/69287762

[Delivers #69287762]
Merge pull request #126 from alphagov/backdrop-links-on-data-sets-list-page

Added links to Backdrop to data-set list page
Improve the Bearer token help text

It no longer accurately reflected our process.
Merge pull request #127 from alphagov/improve-bearer-token-help-text

Improve the Bearer token help text
fix typo
Merge pull request #128 from alphagov/improve-bearer-token-help-text

fix typo
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