* make spacer utils responsive by grid tier
* update scale to add two levels, document them
* change responsive spacing utils to avoid the xs abbreviation in the class name for that tier
* update code snippet to match source
* update usage in our docs
* linter
* docs updates

* Fix #17964

Some browsers are lazy when updating dom elements after transition effects. This can be fixed by reading element properties such as offsetHeight or offsetWidth. However, creating a function using the Function constructor just to access such element, results in a violation of Content Security Policy (where applied), which in turn crashes the application. This fix actually reverts to the way this was handled in v3 and should work as intended.

Firefox requires a width and flex-basis value to size the input field correctly. Despite the width being 1%, the input will size correctly at all parent widths.

http://codepen.io/zalog/pen/bpMJmv

floated content has to come first in the DOM, otherwise you can get alignment bugs. in flexbox though, that's not the case. as such, i'm adding this example code to guide folks to the changes needed to move between default and flex modes.

Merge branch 'v4-flex-list-group' of https://github.com/rosskevin/bootstrap into rosskevin-v4-flex-list-group

Fixes #20975: navbar-text on navbar-dark and bg-inverse is invisible

Horizontal form validation example

[Fix #17400] Add tests about padding restoration

Alphabetized carousel options & added "ride" to it

Zero out the default margin-bottom on label.custom-file

Re-use 'between' mixin for 'only' mixin

Add some properties to the list of property order in .scss-lint.yml

Visibility mixin

Don't clearfix if flexbox is enabled. Fixes #18849

Proper input group sizing and alignment in flexbox mode

Add a min-height: 1.5rem; to custom controls

Calculcate the top offset for custom control indicators