Drop .nav-header for .dropdown-header

* Since we no longer have `.nav-list`, I've nuked the `.nav-header`
* However, dropdowns can still have section headings, so I've kept that
around with the addition of `.dropdown-header`
* Restyled it a smidge as well

docs/assets/css/bootstrap.css

@@ -2275,6 +2275,14 @@ input[type="button"].btn-block {
   outline: 0;
 }
 
+.dropdown-header {
+  display: block;
+  padding: 3px 20px;
+  font-size: 12px;
+  line-height: 1.428571429;
+  color: #999999;
+}
+
 .dropdown-backdrop {
   position: fixed;
   top: 0;
@@ -2303,11 +2311,6 @@ input[type="button"].btn-block {
   margin-bottom: 1px;
 }
 
-.dropdown .dropdown-menu .nav-header {
-  padding-right: 20px;
-  padding-left: 20px;
-}
-
 .list-group {
   padding-left: 0;
   margin-bottom: 20px;
@@ -2710,17 +2713,6 @@ button.close {
   text-align: center;
 }
 
-.nav-header {
-  display: block;
-  padding: 3px 15px;
-  font-size: 11px;
-  font-weight: bold;
-  line-height: 1.428571429;
-  color: #999999;
-  text-shadow: 0 1px 0 rgba(255, 255, 255, 0.5);
-  text-transform: uppercase;
-}
-
 .tabbable:before,
 .tabbable:after {
   display: table;