* Clean up some utilities
- Align CSS properties
- In `_spacing.scss`, we had a comment saying what 'a' was for, but we removed that so this comment no longer applies
* Remove '-xs' from `.navbar-toggleable-xs` and remove duplication
* Fix outdated classes to use newer ones
* Update clearfix to use block instead of table display (also reorder properties for linting)
* update docs snippet for clearfix mixin—was apparently still using Less syntax and had old clearfix hack (even before the block change in this PR)
* Clean up _flex.scss a little
This commit just cleans up the formating of _flex.scss by changing
it so that it uses a single `@include media-breakpoint-up` instead
of multiple. It also aligns all of the CSS properties so it looks
a bit nicer.
* Remove `-xs` from flex classes
* fix form-inline with flex enabled
* grunt
* fix alignment of labels
* shorter if syntax
* add new form example to docs for now
* update inline form docs usage guidelines
* responsive margins
* better margin utils
* fix sizing of .form-check
* flexbox alignment of .form-check
* no need to change direction
* support custom controls in inline form, for default and flex modes
* add example of custom select and checks to docs
* remove hidden and visible label variants since we cover that in the usage guidelines at the start and include hidden labels everywhere
* use property value instead of layout name
* apply to all labels
* add a visible label, space it out
* add id
* Add some flexbox nav components
- Includes .nav-justified for inline, tab, and pill nav components
- Includes example of using the flex utils (.d- and .flex-items-) for centered nav
* redo heading hierarchy a bit
