Query builder should allow selecting sha(s)

[jgraham]

For web-platform-tests/wpt@fbc56f3 we wanted to compare the results against the pirior revision of the wpt repo, to ensure that updating the OS didn't unexpectedly change the test results. Therefore it would be good if the query builder would allow selecting specific sha1s for each selected browser.

[lukebjerring]

#689 covered a SHA for the whole run (now in staging.wpt.fyi), but we'll need to follow up with having a single SHA that's per-product instead, which is collapsed + expanded the same way shared channel labels are.

[foolip]

@Hexcles @stephenmcgruer this came up in discussion on #testing today again. If you construct the URL https://wpt.fyi/results/?product=chrome%404a0bac098f&product=chrome%405d6a919353 by hand, there's no way to edit it in the UI.

Aside: do we think it's weird that all the stuff you can use to get to a specific run is in a param called product?