Flexbox all the time (Drop IE9 support and remove $enable-flex option) (#21389)

* 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

docs/components/navs.md

@@ -212,11 +212,7 @@ Add dropdown menus with a little extra HTML and the [dropdowns JavaScript plugin
 </ul>
 {% endexample %}
 
-## Flexbox variations
-
-When in [flexbox mode]({{ site.baseurl }}/getting-started/flexbox/), tabbed and pilled navigation components gain access to additional nav styles. **These aren't available in default Bootstrap** due to a bug in table styles and responsive behavior.
-
-### Justified nav
+## Justified nav
 
 Create equal-width links in a navigation component by adding `.nav-justified` to a `.nav` component. This works with the inline, tab, and pill variants.
 
@@ -277,9 +273,9 @@ And pills, too:
 </ul>
 {% endexample %}
 
-### Centered nav
+## Centered nav
 
-Using our [flexbox utilities]({{ site.baseurl }}/layout/flexbox-grid/#horizontal-alignment), you can also customize your navigation components to change the alignment of nav items. For example, here are center aligned links on the inline nav component.
+Using our [flexbox utilities]({{ site.baseurl }}/layout/grid/#horizontal-alignment), you can also customize your navigation components to change the alignment of nav items. For example, here are center aligned links on the inline nav component.
 
 {% example html %}
 <ul class="nav nav-inline d-flex flex-items-center">