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/_input-group.scss

@@ -4,54 +4,34 @@
 
 .input-group {
   position: relative;
+  display: flex;
   width: 100%;
 
-  @if $enable-flex {
-    display: flex;
-  } @else {
-    display: table;
-    // Prevent input groups from inheriting border styles from table cells when
-    // placed within a table.
-    border-collapse: separate;
-  }
-
   .form-control {
     // Ensure that the input is always above the *appended* addon button for
     // proper border colors.
     position: relative;
     z-index: 2;
+    flex: 1 1 auto;
+    // Add width 1% and flex-basis auto to ensure that button will not wrap out
+    // the column. Applies to IE Edge+ and Firefox. Chrome do not require this.
+    width: 1%;
+    margin-bottom: 0;
+
     // Bring the "active" form control to the front
     @include hover-focus-active {
       z-index: 3;
     }
-
-    @if $enable-flex {
-      flex: 1 1 auto;
-      // Add width 1% and flex-basis auto to ensure that button will not wrap out
-      // the column. Applies to IE Edge+ and Firefox. Chrome do not require this.
-      width: 1%;
-    } @else {
-      // IE9 fubars the placeholder attribute in text inputs and the arrows on
-      // select elements in input groups. To fix it, we float the input. Details:
-      // https://github.com/twbs/bootstrap/issues/11561#issuecomment-28936855
-      float: left;
-      width: 100%;
-    }
-    margin-bottom: 0;
   }
 }
 
 .input-group-addon,
 .input-group-btn,
 .input-group .form-control {
-  @if $enable-flex {
-    // Vertically centers the content of the addons within the input group
-    display: flex;
-    flex-direction: column;
-    justify-content: center;
-  } @else {
-    display: table-cell;
-  }
+  // Vertically centers the content of the addons within the input group
+  display: flex;
+  flex-direction: column;
+  justify-content: center;
 
   &:not(:first-child):not(:last-child) {
     @include border-radius(0);
@@ -60,9 +40,6 @@
 
 .input-group-addon,
 .input-group-btn {
-  @if not $enable-flex {
-    width: 1%;
-  }
   white-space: nowrap;
   vertical-align: middle; // Match the inputs
 }
@@ -167,11 +144,8 @@
   // element above the siblings.
   > .btn {
     position: relative;
-
-    @if $enable-flex {
-      // Vertically stretch the button and center its content
-      flex: 1;
-    }
+    // Vertically stretch the button and center its content
+    flex: 1;
 
     + .btn {
       margin-left: (-$input-btn-border-width);