Merge pull request #11790 from twbs/drop_recess_for_less

Drop grunt-recess for grunt-contrib-less
enable CSS source maps in grunt-contrib-less
improve compliance with LESS's strictMath
Merge pull request #11807 from twbs/stricter-math

improve compliance with LESS's strictMath
add parens around arithmetic in recursive mixin calls to comply with LESS strictMath

<3 @seven-phases-max for helping to confirm & debug this
X-Ref: https://github.com/less/less.js/issues/1725
misc strictMath compliance fixes
Merge pull request #11808 from twbs/strict-math-fixes

more LESS strictMath fixes
enable strictMath in grunt-contrib-less config
Merge pull request #11809 from twbs/enable-LESS-strictMath

enable strictMath in grunt-contrib-less config
Remove unused variables.
Buildprocess: use module "load-grunt-tasks" to load grunt tasks directly from the package.json
correct indentation
hide space characters in input-group-btn to fix segmented buttons double border
Update less.js to v1.5.1.

This fixes an error with `&:extend(.clearfix all)` when using the customizer.
transform origin mixin for explorer
Merge pull request #11812 from tlindig/load_grunt_tasks

Build process: use module "load-grunt-tasks" to load grunt tasks
Merge pull request #11818 from XhmikosR/less.js

Update less.js to v1.5.1.
Allow listbox AIRA role on dropdown component.
Update README.md.

Mention that Python needs to be in PATH for Pygments to work and thus jekyll not fail.
README.md: mention an alternative workaround for the jekyll issue on Windows.