(less) Better clearing on .dl-horizontal for empty dd elements

Instead of clearing the entire `.dl-horizontal`, we've moved the
clearfix mixin to the `dd` level to solve two problems in one: empty
`dd`s mid-list, and empty `dd` as the last child.

Addresses a few old issues, including twbs#2824, twbs#3819, twbs#3821, twbs#4062,
twbs#6707, and twbs#7180.

Sorry that took so long :D.

type.less

@@ -149,7 +149,6 @@ dd {
 }
 // Horizontal layout (like forms)
 .dl-horizontal {
-  .clearfix(); // Ensure dl clears floats if empty dd elements present
   dt {
     float: left;
     width: (@component-offset-horizontal - 20);
@@ -158,6 +157,7 @@ dd {
     .text-overflow();
   }
   dd {
+    .clearfix(); // Clear the floated `dt` if an empty `dd` is present
     margin-left: @component-offset-horizontal;
   }
 }