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

1 files changed, 21 insertions, 23 deletions

diff --git a/scss/utilities/_flex.scss b/scss/utilities/_flex.scss
index c3efb9d71..4d13e9f8c 100644
--- a/scss/utilities/_flex.scss
+++ b/scss/utilities/_flex.scss
@@ -2,32 +2,30 @@
 //
 // Custom styles for additional flex alignment options.
 
-@if $enable-flex {
-  @each $breakpoint in map-keys($grid-breakpoints) {
-    @include media-breakpoint-up($breakpoint) {
-      $infix: breakpoint-infix($breakpoint, $grid-breakpoints);
+@each $breakpoint in map-keys($grid-breakpoints) {
+  @include media-breakpoint-up($breakpoint) {
+    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);
 
-      // Flex column reordering
-      .flex#{$infix}-first     { order: -1; }
-      .flex#{$infix}-last      { order: 1; }
-      .flex#{$infix}-unordered { order: 0; }
+    // Flex column reordering
+    .flex#{$infix}-first     { order: -1; }
+    .flex#{$infix}-last      { order: 1; }
+    .flex#{$infix}-unordered { order: 0; }
 
-      // Alignment for every item
-      .flex-items#{$infix}-top    { align-items: flex-start; }
-      .flex-items#{$infix}-middle { align-items: center; }
-      .flex-items#{$infix}-bottom { align-items: flex-end; }
+    // Alignment for every item
+    .flex-items#{$infix}-top    { align-items: flex-start; }
+    .flex-items#{$infix}-middle { align-items: center; }
+    .flex-items#{$infix}-bottom { align-items: flex-end; }
 
-      // Alignment per item
-      .flex#{$infix}-top    { align-self: flex-start; }
-      .flex#{$infix}-middle { align-self: center; }
-      .flex#{$infix}-bottom { align-self: flex-end; }
+    // Alignment per item
+    .flex#{$infix}-top    { align-self: flex-start; }
+    .flex#{$infix}-middle { align-self: center; }
+    .flex#{$infix}-bottom { align-self: flex-end; }
 
-      // Horizontal alignment of item
-      .flex-items#{$infix}-left    { justify-content: flex-start; }
-      .flex-items#{$infix}-center  { justify-content: center; }
-      .flex-items#{$infix}-right   { justify-content: flex-end; }
-      .flex-items#{$infix}-around  { justify-content: space-around; }
-      .flex-items#{$infix}-between { justify-content: space-between; }
-    }
+    // Horizontal alignment of item
+    .flex-items#{$infix}-left    { justify-content: flex-start; }
+    .flex-items#{$infix}-center  { justify-content: center; }
+    .flex-items#{$infix}-right   { justify-content: flex-end; }
+    .flex-items#{$infix}-around  { justify-content: space-around; }
+    .flex-items#{$infix}-between { justify-content: space-between; }
   }
 }