Give better caching directives for static files in web interface

[Nicolas]

It looks like my web browser is requesting /default.css and /bg_gradient.jpg every single time I switch to a new page. Even if I use back and forward buttons, which don't cause the page to be requested again, it still requests again those two files.

The requests properly use If-Modified-Since, and buildbot properly replies with 304 Not Modified, but it's still annoying. Clutters the webserver log, for one.

According to  redbot, the responses are “stale” as soon as they go out. They should have an appropriate Expires header, or Cache-Control: max-age=3600 (1 hour expiration from now).

[marcusl]

Right, we should probably add a 'media' directory where static content go (css/images) and export all that with an expires header.

Since I run buildbot proxied behind apache, I add expires headers in apache's config file, but that wont work when running twistd.

[marcusl]

I'm not really sure how to fix this a.t.m. so I'm unaccepting it until myself or someone else tackles it properly.

[dustin]

This will be even more important when serving a lot of static JS