Adding role attribute selector to top bar dividers

Looking through some aria documentation, I noticed there was a role for separators within menus. Adding the role would help screen readers realize the .divider li elements are not part of the actual menu outline. Writing li class="divider" role="separator" everywhere seemed redundant, however. This change adds a [role="separator"] selector to the existing .divider selectors, allowing the markup to be shortened to simply li role="separator".