[Cashtab] Improve documentation

Summary:
A few related improvements to make Cashtab more accessible for devs.

1. We script the complicated prep and installation of local dependencies
2. We update the README to reference the script (and remove obsolete stuff)
3. We add a .env.example file to show what env vars are used by Cashtab.

Test Plan: proofread, follow quickstart instructions in readme

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18459