Reveal BCD ID in rendered HTML

[peterbe]

Fixes #1238

I just made the change without knowing how to test this in kuma. Also, I don't know if identifier is the right string :)
But the goal is to be able to do something like this:

from pyquery import PyQuery as pq
doc = pq("https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video")
for wrapper in doc('div.bcd-data').items():
    print(wrapper.data('bcd-id'))
    break
else:
    print("Has no BCD data table!")

and that should print the identifier that is equivalent to what we put in the front-matter of https://github.com/mdn/stumptown-content/blob/master/content/html/reference/elements/video/docs.md

PS. I don't know how to write and run unit tests for KS.

[peterbe]

The context for this is to be able to use the Document JSON API and get that identifier out. The Document JSON API has a key which is the rendered body HTML which we can parse and pick apart.
The other objective is the scrape-mdn.js project in stumptown-content which needs this identifier to be able to create the right front-matter from an MDN URL.

[peterbe]

I tested this manually now. In Kuma and sure enough it's there:

<div class="bc-data" id="bcd:html.elements.details">

[peterbe]

FYI @escattone perhaps we should set up that thing we have in kuma so that we always use the "Squash and rebase" thing.