* 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
* Explore responsive display utils, but with a twist: lowest breakpoint has no breakpoint modifier in the class name
* make floats use the same format, add float-none mixin
* 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
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.
