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

scss/_media.scss

@@ -1,36 +1,14 @@
-@if $enable-flex {
-  .media {
-    display: flex;
-  }
-  .media-body {
-    flex: 1;
-  }
-  .media-middle {
-    align-self: center;
-  }
-  .media-bottom {
-    align-self: flex-end;
-  }
-} @else {
-  .media,
-  .media-body {
-    overflow: hidden;
-  }
-  .media-body {
-    width: 10000px;
-  }
-  .media-left,
-  .media-right,
-  .media-body {
-    display: table-cell;
-    vertical-align: top;
-  }
-  .media-middle {
-    vertical-align: middle;
-  }
-  .media-bottom {
-    vertical-align: bottom;
-  }
+.media {
+  display: flex;
+}
+.media-body {
+  flex: 1;
+}
+.media-middle {
+  align-self: center;
+}
+.media-bottom {
+  align-self: flex-end;
 }