Revamp header to be more responsive

Also fix navbar
Make header areas visible when small
Don't bother with download links on xs
Make footer more responsive
Make filters box more responsive
Remove margin from articles
Set height on header row, make bookends look better
Use mixins where we can
Only set height if navbar not collapsed
Center collaborator links
Collaborator links open in new tab
Fix footer height and responsiveness
Don't bother with xs columns
Fix leaflet-left margins