Fixes #10370: Improved and deprecated dropdown menu alignment options

* Removes an old pair of selectors that didn’t properly target the
right-aligned navbar alignment of dropdown menus.
* Deprecates the `.pull-right` alignment in favor of a more specific
and unique class name.
* Adds `.dropdown-menu-right` as the new alignment class. This is then
mixin-ed into the `.navbar-right.navbar-nav` dropdown menus for
auto-alignment.
* To override that auto-alignment, use `.dropdown-menu-left` as needed.

less/dropdowns.less

@@ -46,6 +46,8 @@
   background-clip: padding-box;
 
   // Aligns the dropdown menu to right
+  //
+  // Deprecated as of 3.1 in favor of `.dropdown-menu-[dir]`
   &.pull-right {
     right: 0;
     left: auto;
@@ -126,6 +128,25 @@
   }
 }
 
+// Menu positioning
+//
+// Add extra class to `.dropdown-menu` to flip the alignment of the dropdown
+// menu with the parent.
+.dropdown-menu-right {
+  left: auto; // Reset the default from `.dropdown-menu`
+  right: 0;
+}
+// With v3, we enabled auto-flipping if you have a dropdown within a right
+// aligned nav component. To enable the undoing of that, we provide an override
+// to restore the default dropdown menu alignment.
+//
+// This is only for left-aligning a dropdown menu within a `.navbar-right` or
+// `.pull-right` nav component.
+.dropdown-menu-left {
+  left: 0;
+  right: auto;
+}
+
 // Dropdown section headers
 .dropdown-header {
   display: block;
@@ -180,7 +201,12 @@
 @media (min-width: @grid-float-breakpoint) {
   .navbar-right {
     .dropdown-menu {
-      .pull-right > .dropdown-menu();
+      .dropdown-menu-right();
+    }
+    // Necessary for overrides of the default right aligned menu.
+    // Will remove come v4 in all likelihood.
+    .dropdown-menu-left {
+      .dropdown-menu-left();
     }
   }
 }