LP#1681095 Browser cache-busting key for longer expires

LP#1681095 Browser cache-busting key for longer expires

Extend the support for cache-busting that we added with autogen's generation of
ctx.eg_cache_hash for core JavaScript assets to other assets such as images,
stylesheets, and additional JavaScript. This will enable us to set very long
cache times for our assets, which can then be controlled explicitly by setting
eg.cache_key in config.tt2, or by default any time you run autogen.

Addresses TPAC and KPAC

Signed-off-by: Dan Scott <dan@coffeecode.net>