Show a list of repos on non-assets 404 requests

When the Bosco server returns a 404 that is not an asset request (requests that do not end in .css, .js, .png, etc) we are going to serve a page with the list of repos included in the service.

Descriptions, tags and path for each of these repos are shown for easier navigation across all repos. See demo below.

![bosco repo demo](https://raw.githubusercontent.com/gabceb/bosco/repo-404-gif/c46y5.gif)

Extra bonus:

- Ability to filter repos by tags, description and name
- Ability to copy the repo path to the clipboard. (Note: I was just playing with this feature but I am not completely convinced it should be there especially since it will break for users with no-flash. I wont be heartbroken if it is taken out.

I will work on testing once this PR is accepted.