[Cashtab] Prevent caching of index.html

Summary:
T2459

Prevent caching of index.html

This ensures that web app users will always get the most recent version of the app. The JS bundle changes file name with each deployment, so loading the latest index.html will also load the latest js bundle.

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

Confirm output includes:

Cache-Control: max-age=0, no-cache, no-store, must-revalidate
Pragma: no-cache

Navigate to localhost:8080 and inspect the Network tab in the developer console to get the filename of the js bundle (it probably will not be what I have below, since it changes with every build). Then,

curl -I http://localhost:8080/static/js/main.ca0925d3.js

Confirm output does not include the above cache statements (we want js files to be cached).

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11592