Write Glyphicons to _data, generate Glyphicons markup

This generates a glyphicons.yml file from the glyphicons.less source,
and then components.html uses that data to generate the Glyphicons docs
by iterating through the data. The _data directory was introduced in
Jekyll 1.3.0, and GitHub Pages is already on [version 1.4.2][1].

Fixes #11862.

[1] https://github.com/github/pages-gem/blob/master/github-pages.gemspec#L16