Add long-term caching headers for media and static assets

[damz]

Interestingly, long-term caching for media and static assets was never enabled by default. Enabling this has a dramatic effect on the user experience, especially in the admin area. This is only safe when static content signing is enabled, but it is current force enabled as part of the ECE tools.

[JacobBrownAustin]

This looks good to me.
I think there was some confusion about this because there's two conflicting things I've heard since I started on this team.
A) Static files are cached for a very long time because they are versioned.
B) Browser caching is disabled because Fastly is so good that we don't need browser caching at all.

B Doesn't really make sense because even if Fastly is the best thing ever, it's still not as quick as local filesystem.

[shiftedreality]

@damz can't we use max instead of 1y?

[damz]

B) Browser caching is disabled because Fastly is so good that we don't need browser caching at all.
B Doesn't really make sense because even if Fastly is the best thing ever, it's still not as quick as local filesystem.

Yes, the correct statement should actually be:

B) Browser caching is disabled for mutable resources so that invalidation takes effect immediately.

Immutable resources (i.e. static assets, media) should be infinitely cached.

can't we use max instead of 1y?

No, our parser expects a proper duration here.