yelp-xsl wip/html5 -> master

[Shaun McCance <shaunm gnome org>]

Finally, I have merged the wip/html5 branch of yelp-xsl to master.
There will have to be updates to yelp and yelp-tools in this release
cycle to match.

If you have customizations on top of yelp-xsl (for example, that you
pass to yelp-build or pintail), you will almost certainly need to
change them. This is true even if you just changed some CSS. Here's an
incomplete summary of some of the changes:

* Various divs have been replaced with semantic HTML5 elements like
article, section, and nav.

* The main page layout uses two flexboxes for layout, the outer
vertical and the inner horizontal.

* In general, quite a few layouts have been switched to flexbox. For
example, image tiles are now flexbox instead of inline-block.

* Horizontal max-width and centering happens on more deeply nested
elements, making it easier to do certain customizations.

* There are no more external images. It's all inline SVG, stylable with
CSS FTW.

* jQuery has been removed.

* Syntax highlighting has been switched to highlight.js (away from
jQuery.Syntax). Colorizers are now included by default.

* Speaking of colors, the themes color names are different and more
extensive. Most of the colors can be computed automatically using color
algorithms that probably never should have been written in XSLT.


Please pass this information along to anybody you think might be
affected by these changes.

Thanks,
Shaun