Removing margins from :first-child / :last-child navbar-form

Navbar Forms that are first or last children **and** pulled right or left currently have left and right margins.

.navbar-nav.navbar-left, and .navbar-nav.navbar-right currently strip this margin away and allow elements within to justify to the left or right.

This update allows buttons in navbar-forms to align with other header / footer elements and when fixed to the top or bottom, also align with the elements on the page.