Signin/logout buttons added to navbar on mobile layout

Only one collapse container can be added to navbar (There's only
one navbar to be collapsed!). Note the Bootstrap example doesn't collapse
properly on mobile layout mode. Also note the extra right container to
be collapsed is **inside** other collapsing container.

Instead use a `<div class="nav navbar-right"></div>` container, letting
it be collapsed just being inside the parent collapsing container.