Ask for a long max-age on the iframes, cachebust it with the build id (bug 1087399)

https://bugzilla.mozilla.org/show_bug.cgi?id=1087399
- Changed the duration to one year - also, yeah, both `Expires` and `Cache-control: max-age=` have the same effect, but both are automatically set by django anyway when you use the `cache_control` and `patch_cache_control` functions we are using.

- Used `urlparams()` to handle query string issues

- Added a comment