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/_list-group.scss

@@ -3,10 +3,8 @@
 // Easily usable on <ul>, <ol>, or <div>.
 
 .list-group {
-  @if $enable-flex {
-    display: flex;
-    flex-direction: column;
-  }
+  display: flex;
+  flex-direction: column;
 
   // No need to set list-style: none; since .list-group-item is block level
   padding-left: 0; // reset padding because ul and ol
@@ -19,15 +17,10 @@
 // Use on `li`s or `div`s within the `.list-group` parent.
 
 .list-group-item {
-  @if $enable-flex {
-    display: flex;
-    flex-flow: row wrap;
-    align-items: center;
-  } @else {
-    display: block;
-  }
-
   position: relative;
+  display: flex;
+  flex-flow: row wrap;
+  align-items: center;
   padding: $list-group-item-padding-y $list-group-item-padding-x;
   // Place the border on the list items and negative margin up for better styling
   margin-bottom: -$list-group-border-width;