Plumb diff filter params through the UI

[lukebjerring]

Description

Splits path param from filter for parsing, and plumbs the value to the webcomponent.
We'll need this to make the UI ignore tests that didn't run on a PR.

Also changes the /api/diff endpoint to allow anything that produces exactly 2 results, e.g. ?product=chrome&max-count=2

Review Information

Load some filtered view, e.g. /results?products=chrome[experimental],chrome[stable]&diff&filter=U will only show unchanged things (kinda useless, but easy to tell it works).

[wpt-pr-bot]

Staging instance deployed by Travis CI!
Running at https://diff-filter-dot-wptdashboard-staging.appspot.com

[wpt-pr-bot]

Staging instance deployed by Travis CI!
Running at https://diff-filter-dot-announcer-dot-wptdashboard-staging.appspot.com

[foolip]

Confirmed that /results?products=chrome[experimental],chrome[stable]&diff&filter=U does something.

[foolip]

LGTM % https://diff-filter-dot-wptdashboard-staging.appspot.com/results/?products=chrome[experimental],chrome[stable]&diff&filter=C including some "0 / 0 / 0" rows, but that'll be fixed in a follow up.

[foolip]

This is nicer now :)

[lukebjerring]

Yeah, as discussed offline,

Take beacon-basic-blob.html for example. The UI shows it as 0 / 0 / 0, but if you see the /api/diff fetch:
https://diff-filter-dot-wptdashboard-staging.appspot.com/api/diff?before=chrome%5Bexperimental%5D&after=chrome%5Bstable%5D&filter=C you don't see beacon-basic-blob.html in the result.

So, as you say, I'll be improving the client-side filtering to match the same behaviour in the UI.