Clean up nav markup a wee bit.
Updated navigation styling on mobile to match "white" desktop
appearance.
Re-added separate tablet and mobile social link markup.

Added some better documentation so we don’t forget what that’s for.
Cleaned up link styles in footer to be more consistent.
Little bit more documentation.
Remove extraneous display blocks.
Merge pull request #315 from DoSomething/nav-cleanup

Clean up navigation styles a wee bit.