Add route for looking up users by dataset name

- Doing name for now, not data-group/type, as that seems to be the way we're implementing our routes at the moment and I didn't want to get into that refactor/addition in the scope of this story
- Implementation: `data-sets/some-data-set/users` brings back the users assigned to `some-data-set`
Stub method for doing user lookups by passing data-set name
Return a `user_api_object`

- At the moment this will just be an email, but I thought wrapping up an api object in the model might serve as a good guide for passing back a partial version of the backdrop user object
- Still using name
Merge pull request #176 from alphagov/dataset-ownaz

Add a view for looking up users by data-sets
Remove Varnish purging code

We have removed the long caching of stagecraft api responses and do not
plan to re add it so the varnish purging code is no longer necesarry.
Merge pull request #177 from alphagov/remove-varnish-purge-code

Remove Varnish purging code
Readme explaining the new migration script moving to schemas
Update docs around making requests

- includes a live example that shows we need the signon api key, not the tokens that exist inside pp-deployment (are these now completely redundant?)
- Updates example json responses to show off the exciting new user listings
Merge pull request #180 from alphagov/document_users

Update docs around making requests
Merge pull request #179 from alphagov/docs-for-data-set-schema-change-script

Readme explaining the new migration script moving to schemas
Remove backdrop dependency for data set creation

Backdrop will create data sets on it's side as they're written to now
the dependency on backdrop can be removed from stagecraft for data set
creation.

This avoids the cross app transaction bugs around creating data sets.

The dependency has not been removed for data set deletion. It is already
a much more resilient process because if backdrop doesn't know a about a
data set stagecraft just caries on.
Merge pull request #178 from alphagov/remove-backdrop-dependency

Remove backdrop dependency for data set creation