Add missing thing

Running the tests was giving me warnings about this not being
available.
Initial draft of write authorization

Needs more tests and work around deployment / revocation,  but
contains the general idea.
Add headers when using HTTPTestClient

So headers such as the bearer token are passed onto the apis.

@maxfliri
@robyoung
Add reptiles bucket to test bearer tokens

@maxfliri
@robyoung
Move tokens out into their own config files

This better reflects how they are deployed. Tokens change per
organisation but general configuration does not.

@maxfliri
@robyoung
Merge pull request #43 from alphagov/write-authorization

Write authorization