Restyled topbar to resemble UX guidelines

Also moved topbar styling into separate component
Adjusted context selection to match other dropdowns, merged it with topbar styles file

Partially-Implements: blueprint css-breakdown
Partially-Implements: blueprint navigation-styling
Change-Id: If30b85f5415a3e8d0416fdafeb233b722feb014f

Conflicts:
	openstack_dashboard/static/dashboard/img/logo.png