Improve 404 Page

[vkoves]

The current indigoBox 404 page is unstyled and unprofessional, and I believe is a remnant from an older state of the website. The copy should be updated to be more in line with our brand, including the title, which is currently Page Not Found :(.

For reference, here are some good examples of 404 pages:

• GitHub's delightful 404 page
• MailChimp's 404 page

In the most basic case, we can do something like Carpe's 404 page. It's not gorgeous, but it is good enough for the time being, and is much better than the current indigoBox 404 page.

[vkoves]

In commit a8050c4 I started some improvements to the 404 page, in particular making it use the indigoBox header and be reformatted a bit. @AcademyInPeril - please take a look at the text and provide revisions. @rjaltman - what do you think we could do with this stylistically?

Here's a screenshot of the new version: