Milo should support linking to Travis-like badges for specific builders so that repositories can include their build status in their front mages (e.g., README.md).

I'm thinking something like this: for each builder, add a URL extension that resolves to a "passing", "failing", or "unknown" status for a builder based on its latest build.

Proposed URL:
- For builder: https://ci.chromium.org/p/infra/builders/luci.infra.ci/luci-gae-continuous-trusty-64
- Have a URL: https://ci.chromium.org/p/infra/builders/luci.infra.ci/luci-gae-continuous-trusty-64/ci-status-badge.png

The ".png" URL would resolve into a static resource URL based on the status of the most recent build.

Travis badges can be found here:
- https://raw.githubusercontent.com/travis-ci/travis-api/master/public/images/result/passing.png
- https://raw.githubusercontent.com/travis-ci/travis-api/master/public/images/result/failing.png
- https://raw.githubusercontent.com/travis-ci/travis-api/master/public/images/result/unknown.png

* Probably should mod them a bit for copyright reasons - maybe "LUCI:passing"?