[Tails-dev] New build option: cachewebsite

Hi,

the new "cachewebsite" Tails build option enables caching of the
built website.

The cache is keyed on the input parameters that primarily determine
the output of the website build, as a trade-off between build
performance and build result correctness.

Once we're confident enough, we can enable this option by default
and make our CI benefit from it as well.

So please try it out and report any problem you see!

For usage documentation, see wiki/src/contribute/build.mdwn on the
stable branch.

Cheers!