Site cleanup

I set out to do a quick and dirty typography overhaul to make things
more consistent and a bit cleaner, but I kept running into things I
didn't like, so this lead me down something of a rabbit hole of design
tweaks to the OSM site.

Goals here are to have better content hierarchy, better vertical
rhythm, more consistent UI, cleaner markup with less tables, all while
keeping the basics pretty much intact. There are a lot of things I
didn't touch even though they need to be updated (lots of tables where
stuff shouldn't be tables, mostly).

Basic overview of changes:

I added a new persistent header that helps segment pages. It's now
a lot easier to know what you're looking at. The header has a page
title, a description, plus a submenu of actions.

There is now a pattern of 20px/10px margins and padding for more
rhythm and breathing room throughout the site.

I know there are other problems here or things I've missed - would
love another set of eyes to look over this! I am still getting comfortable
working on the site - it's my first time ever digging into a ruby or rails
so I'm not familiar with the templating language yet.