Prefetch dialog components from the communication_iframe

When the browser is idle, it will start fetching and caching the
dialog's Javascript, CSS and images. This works on IE9 and Firefox
(it's currently disabled in Chromium).

The trick is to make sure that the communication iframe is open
while the user is busy reading the RP's site. When using the
Observer API, this happens automatically at the end of the
navigator.id.watch() call.
Make use of new connect-cachify helper function
Specify the locale when prefetching the dialog stylesheet
Only emit prefetch tags in production mode

There are far too many JS/CSS files to prefetch in development
mode for prefetch to be effective (browsers seem to put a limit on
the number of files they will prefetch).
Merge pull request #3273 from fmarier/prefetch-in-iframe-newcachify

Prefetch dialog resources in the communication_iframe

Sauce failures are due to other issues - opened PR #3388 to fix them. When merged, this PR works beautifully.

r+, I dig this. Nice work @fmarier.