Ahead-of-time GZIP compression

[rauchg]

Upon next build. This is for the next major.

• For main build files - Add AOT gzip content-encoding support for main build files. #565
• For JSON pages - Add Gzip support for JSON pages #571
• Allow to extend compression via next.config.js- Add support for customizing compress encodings #657

[arunoda]

So, we detect the accept-gzip header and server gzipped version if so.
Okay, that's a good idea.

[timneutkens]

Closing this in favor of #984