Use single quotes in bearer token error

Using double quotes means that they have to be escaped in the JSON
output that we return to users. It's nicer to use single quotes so
that they're easier to read in the JSON.
Remove RAW_QUERIES_ALLOWED configuration

This hasn't been used since at least 65fe1d3a58dc0b4cad7caba753fd28a4af2fee6e
in September 2013.
Add feature tests for read API cache header times

As Alex,
I would like to have confidence in my refactor,
So that I don't break an application I haven't worked on in months.
Refactor Backdrop to use the client's admin API

The performanceplatform-client now has the ability to interact with
Stagecraft, so replace the concept of DataSetRepository and
UserHttpRepository with that.

Also remove the concept of DataSetConfig and UserConfig because
those namedtuples don't really add anything anymore.

This changes the authentication mechanism from a token that Stagecraft
knows to an OAuth access token for an API user from Signon.
Merge pull request #326 from alphagov/refactor-use-oauth-tokens

Refactor Backdrop to use the Performance Platform client's admin API