Branding

Various branding things:

- added logo to navbar https://www.pivotaltracker.com/story/show/77766812
- switched to sans serif for main body font etc
- added favicon https://www.pivotaltracker.com/story/show/77766840
- added apple icon for iOS users https://www.pivotaltracker.com/story/show/78177652

And while I was in there I added an accessibility link to skip the nav menu, as per https://www.pivotaltracker.com/story/show/73521848