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.

* Fix for right padding in dismissible alerts

* Removed extra whitespace

* Update _alert.scss

* Update _alert.scss

Merge branch 'custom-checkboxes-radios-correct-color' of https://github.com/sabrown84/bootstrap into sabrown84-custom-checkboxes-radios-correct-color

We don't change anything so the transition is basically unused.

* add margin-bottom to .btn of a vertical button group

* match margins of radio/checkbox buttons in a btn-group to the actual buttons

* add comment

This also fixes the wrong usage of `site.description` instead of `page.description` in meta description.

This saves a few bytes from the unminified dist files.