[Cashtab] Run npm test before commit

Summary:
T2439

Implementing husky to prevent git commits if cashtab unit tests fail.

Test Plan:

1. cd web/cashtab && npm i
2. Change the text inside the <ZeroBalanceHeader> component inside Send.js
3. git add .
4. git commit -m '[Cashtab] Test commit to see if tests run'
5. Note that unit tests run and fail as snapshots have changed
6. Note that this commit did not complete

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11566