Implements a fallback on the stagecraft api model fetch method

When this is called it will first attempt to get config from stagecraft
before falling back to a request to the local app.

This is tested by faking sync to record the url used at each stage in
the fallback. There is also a unit test for the fallback toggling
itself.

This still requires pp-deployment changes to set up the per environment
urls
Tidy up for stagecraft api client and tests

makes things a bit clearer.
Fix for multiple errors being triggered

we now ignore the first error in process request. this might not be the
best way to do it but I can't immediately see what option to use on the
first fetch to prevent the error trigger.

This also fixes the tests for this change.
Fixes for Tom's comments, fix linting

Dashboards slugs should not have the root, also, dashboards/public
should be public/dashboards.
Merge pull request #782 from alphagov/dashboard-api-with-fallback

Implements a fallback on the stagecraft api model fetch method