Serve dynamic content using templates

[uhthomas]

There are some limitations with serving static content which hurt customisation and usability. This is partly related to #3, but extends it by tackling some other issues such as:

• Expiration time not matching the origin
• File size limit not matching the origin
• Non-customisable elements such as the abuse email
• Non-customisable name

There are various other things that might be nice to have dynamic, such as the tagline, and even the removal / addition of some client-side features, but this is more about getting the system in place.