Start some nav and dropdown cleanup

* Remove border-radii on top of tab dropdown menus
* Remove the .dropdown and .dropup styles and instead place  on all
* Use  on dropdown carets intead of separately applying margin with a top alignment
* Some other misc cleanup

less/navs.less

@@ -12,8 +12,11 @@
   padding-left: 0; // Override default ul/ol
   list-style: none;
   .clearfix();
+
   > li {
+    position: relative;
     display: block;
+
     > a {
       position: relative;
       display: block;
@@ -44,7 +47,7 @@
       cursor: default;
     }
     // Space the headers out when they follow another list item (link)
-    &+ .nav-header {
+    + .nav-header {
       margin-top: 9px;
     }
   }
@@ -215,6 +218,15 @@
 
 
 
+// Dropdowns
+// -------------------------
+
+.nav-tabs .dropdown-menu {
+  // Remove the top rounded corners here since there is a hard edge above the menu
+  .border-top-radius(0);
+}
+
+
 /*
 // Prevent IE8 from misplacing imgs
 // See https://github.com/h5bp/html5-boilerplate/issues/984#issuecomment-3985989