Input groups overhaul

* Moves input groups CSS into separate file
* Moves input groups docs into the Components page
* Add support for radios and checkboxes in input groups to fix #8679

customize.html

@@ -104,6 +104,13 @@ base_url: "../"
     <h3>Components</h3>
     <div class="row">
       <div class="col-lg-4">
+        <div class="checkbox">
+          <label>
+            <input type="checkbox" checked value="input-groups.less">
+            Input groups
+          </label>
+        </div>
+
         <h4>Navigation</h4>
         <div class="checkbox">
           <label>