[Cashtab] Finalize webpack upgrade

Summary:
T1682

Diff seems enormous with a lot of changes. However, here is what's going on.

• Deleted web/cashtab
• Renamed web/cashtab-v2 to web/cashtab
• Replaced all instances of cashtab-v2 with cashtab
• Updated CONTRIBUTING.md to reflect latest dependencies for working on the cashtab repo

Test Plan:

1. Confirm no missed strings of cashtab-v2

cd web/cashtab
rm -rf node_modules
npm ci
grep -r 'cashtab-v2' .

Should see no output

2. Confirm no broken features

npm start and test all Cashtab features

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11275