[Cashtab] Webpack upgrade part 8, rework absolute imports

Summary:
T1682

Apply jsconfig.json approach for absolute imports. See step 8 in T1682.

Test Plan:

cd web/cashtab-v2
grep -r '@components' .
grep -r '@assets' .
grep -r '@utils' .
grep -r '@hooks' .

Should get no output from any of these commands, confirming the deprecation of previous @ method of absolute imports.

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11248