Add cache headers for emoji files and compress them

[superpoincare]

The latest alpha has svg support for emoji from s.w.org.

However, it doesn't have cache headers set. In addition, compressing them might save a lot of bandwidth (preferably static?).

Thanks for the report, @superpoincare!

I've passed this info onto the Systems team ​here.

I'm going to close this ticket in favour of that thread, as it's a change that can be made outside of the Core development cycle.