Store results of web-platform-tests PRs and allow comparing to master

[foolip]

This would be part of a solution to web-platform-tests/wpt#7475 that doesn't involve https://pulls.web-platform-tests.org/.

Doing web-platform-tests/wpt#10503 first might make this simpler, but it isn't strictly required, we could also get results from Travis.

[zcorpan]

This is blocked on web-platform-tests/wpt#9874

[lukebjerring]

Whether we should do this is up for debate, regarding Travis CI secrets. We are not able to pull from Travis CI after the results report has been produced, so would have to push from Travis CI. This requires basic HTTP authentication (see API docs).

IMHO, it's fine to have a Travis login and secret, and special-case it in the endpoint to be flagged as untrusted in case we need to do anything extra special later in the UI. Note, also, that we could additionally lock that login down, using say, Travis IPs.

I think there's a lot of value added having these runs in our dataset.

[lukebjerring]

@Hexcles, I trust you to make the right decision ;)

[lukebjerring]

This is now a dupe of the implementation-specific #707.