Add remix-serve --immutable to opt-in to immutable caching for everything

[mjackson]

Strategy should be:

• public/build
  • immutable
  • max-age: 1y
• public
  • max-age: 1h

This is to minimize chances of things being busted in production but also provide some reasonable caching in production for other (maybe non-hashed) stuff in public.

Future: we should probably have a flag for remix-serve that dictates how public caching is to be done. Something like remix-serve --immutable if you want to opt-in to immutable caching for everything.

[machour]

First part is fixed by b1ce044.
Leaving this open for the Future enhancement