[cashtab] enable gzip compression when serving cashtab

Summary:
as title.

local testing showed a notable improvement on load time.

Test Plan:

docker build -t cashtab_local .
docker run --rm -p 8080:80 --name cashtab cashtab_local

send and receive tokens and BCH. also the generated Lighthouse report on localhost:8080 in an incognito window shows a ~60% reduced Time to Interactive metric over master.

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D9472