popover dismiss-on-next-click example: instead of <button>, use <a> w/ tabindex

Fixes #14038.

Research supporting this choice of approach:
    https://gist.github.com/cvrebert/68659d0333a578d75372

Why not <button>?
Even with a `tabindex` attribute, they don't gain focus when clicked upon in:
    * Mac OS X Firefox 30.0
    * Mac OS X Safari 7.0.5
regenerate docs/assets/js/docs.min.js
speed up js tests a bit
Tabs to spaces
Merge pull request #14171 from wangsai/tab2space

Tabs to spaces in variables.less
Merge pull request #14055 from twbs/fat-speed-up-tests

speed up js tests a bit
Merge pull request #14156 from twbs/fix-14038

Fix popover dismiss-on-next-click example