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. 

~~__This is not yet complete__ but I wanted to submit it now as a pull request to let people check out what I'm up to. I hope to polish this and have it ready to merge tomorrow or wednesday at the latest.~~

__This is ready to be reviewed__

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 site so I'm not familiar with the templating language yet.