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.
Merge branch 'master' of github.com:twbs/bootstrap