Code review comment for ~pappacena/launchpad:https-mirrors

Thanks for chasing this up! Looks like mostly a good job on the prober changes - just a few small things. Also, you're going to need to make at least basic changes to lib/lp/registry/stories/webservice/xx-distribution.txt and lib/lp/registry/stories/webservice/xx-distribution-mirror.txt due to the newly-exported fields, and it would be good to add suitable tests to lib/lp/registry/browser/tests/distributionmirror-views.txt and lib/lp/registry/doc/distribution-mirror.txt too (acknowledging that the doctests are unpleasant).