Support Quick Docs (Web Platform Docs) in HTML

Enables Quick Docs for HTML.
Data is still from Web Platform Docs, while HTML data is in `html.json` and CSS data in `css.json`.

As there's docs for both tags and attributes, the behaviour is different based on the cursor position.
* If the cursor's on a (supported) tag like div, docs for the tag will be shown.
* If the cursor's on a (supported) attribute like tag (but not on its value!), docs for the attribute will be shown.
* If the cursor's on a unsupported attribute, docs for the tag will be shown (if supported).

Also, there's a new `fullscreen-summary` class applied when there's only a summary to display and nothing else (this can happen for CSS as well, but only for very few properties). But HTML docs only consist of a summary and thus, it would look odd if it was only half-screen.
Also, the summaries are mostly only consisting of one or two lines (that's what a summary is for) and in fact, that's really short. So I guess the main benefit of Quick Docs in HTML is having the "Read more" link for most users (to get things like Examples, syntax, ... that we can't access via their API). But for some users, like for those learning HTML, even such a short summary can be really helpful.

![image](https://cloud.githubusercontent.com/assets/2641501/5220637/60edf9a2-7668-11e4-8f78-6759481c116a.png)

For #9566.