#651 New 404 page

Our 404 page wasn’t looking too good (just the default Rails one), so I just rewrote the whole thing. I added an image of an unhappy lifesaver, but otherwise everything (including all the styling) is self-contained on the page. Here’s what it looks like now:

![New 404 page](https://d1015h9unskp4y.cloudfront.net/attachments/b2d0908f-2de9-4963-8c41-6eeaff7e4372/new-404.png)