* remove the $enable-flex variable option
* remove bootstrap-flex.css dist file and it's grunt task
* remove the separate flex css file for docs; it's all the same now
* remove flexbox docs (porting some to the main grid docs in next commit)
* clean up few grid docs bits to simplify copy, start to mention flexbox
* port relevant flexbox-grid.md content to grid.md
- clean up mixins
- update how it works section
- bring over sizing and alignment sections
* remove the $enable-flex from the options.md page
* update lead paragraph to mention flexbox
* update migration to mention loss of ie9 support
* remove mention of flexbox dist file
* clarify IE support
* making a note
* remove flexbox variant mentions from component docs
- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default
* remove $enable-flex if/else from sass files
* remove flex dist files
* update scss lint property order to account for flex properties
* linting
* change to numberless classes for autosizing, wrap in highlighting div
* bump gruntfile and postcss to ie10
* redo intro sections
* rearrange
* phew, redo hella grid docs
- rearrange all the things
- consolidate some bits
* remove reference to flexbox mode
* more border action for demo
* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom
This pulls in some changes from #18462 to include a set of generic color variables. It doesn't include a Sass map for generating the theme colors yet, since we can't easily do that for all components, but does give a few more color choices to folks and an easier way to customize.
* Darken dismiss icon opacity
* Darken text on active list group items
* Darken light gray for accessibility/contrast, then update gray and dark gray to match
* 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
