Prefix all CSS classes with hxr-

[humitos]

Avoid collisions with other CSS frameworks/themes/etc.

See executablebooks/sphinx-book-theme#577
Closes #180

[humitos]

I opened this PR as an example showing what's the work required to prefix all the CSS classes with hxr-. The only work missing here is to define the prefix somewhere globally and re-use it from a variable or similar, instead of hardcoding it everywhere. In fact, we could make it a extension config if it's required.

[agjohnson]

This looks great!

One thing to keep in mind is that the Sphinx template file pattern will not work if we switch to our standard JS/stylesheet build system. I do see a few areas where this build system would add some features and help clean up the files used by users (plus it's using the same system we use elsewhere). But, we haven't found a pattern yet for post-processing the template variables back into the built assets. This is just a note for the future though, I'm sure there is an option we haven't explored yet.

[humitos]

Good note! I will probably hit that problem when working on the PR to include jQuery into the extension itself: #204. So, I will try to figure that out altogether. If it's not possible, I will probably remove the ability to change the prefix. At the moment, it's not exposed to users at least, which is good.