[Cashtab] Implement CSP header

Summary:
D15106 was deployed with no issues. Cashtab functions worked, also sideshift swap.

So, move from 'report-only' to actually implemented header.

Test Plan:

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

In another terminal,

curl -I http://localhost:8080

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15128