[WIP] Add a custom 404 page.

[toolness]

This contains @tekd's #282 with a simplified commit history:

We also need to do a few things before merging it:

• Run the copy by Batu.
• Consider changing the "click here to go back to the homepage" as hyperlinking words like "here" isn't good for accessibility. Suggestion: just hyperlink "go back to the homepage" instead, and remove the words "click here" entirely.
• Add support for internationalization.

[toolness]

Paging @batusayici, can you take a quick look at the copy above and the accessibility issue?

[chrswng]

going forward we should also run copy past @juliachrusciel too