Cache or eTag resources in /assets folder

[Marc-Andre-Rivet]

We currently eTag / cache resources included in the component repos but do not offer any cache mechanism for resources in /assets. The same mechanism could be applied:

• known assets included in the index_template are fingerprinted using the same versioning / last_modified logic with version=0.0.0
• assets that are not requested through the index_template revert to using eTag

[chriddyp]

@Marc-Andre-Rivet - when I load dash.plot.ly and look at the network console, it looks like assets are cached and have a 304 response. What am I missing?

[chriddyp]

Additionally, the component package JS files look like they aren't getting cached (all of those 200s)