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

less/bootstrap.less

@@ -28,6 +28,7 @@
 
 // Components: common
 @import "component-animations.less";
+@import "input-groups.less";
 @import "dropdowns.less";
 @import "list-group.less";
 @import "panels.less";