`js-compile` and `js-minify` tasks already call their "children" tasks.

We don't support Opera 12, and Firefox's property was the unprefixed one.

Merge branch 'btn_input_split_variables' of https://github.com/isychev/bootstrap into isychev-btn_input_split_variables

1. move a couple of them before the more specific ones.
2. change nesting to be under the same parent selector
3. use the ampersand more

Merge branch 'input-group-custom-forms' of https://github.com/twbs/bootstrap into input-group-custom-forms

The option is set to `always-single-line` in stylelint-config-standard which we extend.

The images are PNG 8-bit optimized with optipng, zopflipng and pngout.