Some previous status manager refactoring broke the badge end point. I've
also changed it from redirecting to the shields.io image, to proxying
it.

If no pipeline_id is found, return a badge with 'not found' written in
it, rather than returning a 404. Knowing that a pipeline is potentially
missing is still useful status.

A status badge at the /badge/collection/<pipeline path> endpoint.

This returns a pipeline collection summary badge, e.g.

pipelines | 22 succeeded, 14 failed, 2 invalid, 1 running

Pipeline path will filter the pipeline ids it is reporting on.

This commit makes application of basic auth (if active) more selective.
Instead of forcing basic auth on all endpoints, it is now applied as a
decorator for the main dashboard views. This allows us to serve the
status badges without basic auth while still being able to protect the
dashboard pages.