[Cashtab] Use monorepo dependencies instead of npm

Summary:
T3548

All monorepo apps with monorepo deps should use the relative install instead of version pinning npm. In this way, we can be sure that upgrades to the libraries are not breaking apps -- and we do not have to do hard-to-remember maintenance diffs to keep all apps on the right version of dependencies all the time.

Test Plan:
./contrib/teamcity/build-configurations.py cashtab-tests

Edit cashtab.Dockerfile so that line 52 reads ARG NGINX_CONF=nginx-preview.conf
docker build -f cashtab.Dockerfile -t cashtab_local .
docker run --rm -p 8080:80 --name cashtab cashtab_local

Navigate to localhost:8080 and Cashtab is built and working

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16125