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/migration.md

@@ -16,7 +16,7 @@ Here are the big ticket items you'll want to be aware of when moving from v3 to
 
 ### Browser support
 
-- Dropped IE8 and iOS 6 support. v4 is now only IE9+ and iOS 7+. For sites needing either of those, use v3.
+- Dropped IE8, IE9, and iOS 6 support. v4 is now only IE10+ and iOS 7+. For sites needing either of those, use v3.
 - Added official support for Android v5.0 Lollipop's Browser and WebView. Earlier versions of the Android Browser and WebView remain only unofficially supported.
 
 ### Global changes
@@ -29,7 +29,7 @@ Here are the big ticket items you'll want to be aware of when moving from v3 to
 
 ### Grid system
 
-- Added support for flexbox (set `$enable-flex: true` and recompile) in the grid mixins and predefined classes.
+- Added support for flexbox in the grid mixins and predefined classes.
 - As part of flexbox, included support for vertical and horizontal alignment classes.
 - Overhauled grid mixins to merge `make-col-span` into `make-col` for a singular mixin.
 - Added a new `sm` grid tier below `768px` for more granular control. We now have `xs`, `sm`, `md`, `lg`, and `xl`. This also means every tier has been bumped up one level (so `.col-md-6` in v3 is now `.col-lg-6` in v4).