[cashtab] Update file-loader and fs-extra

Summary:
The current versiond epend on outdated version of webpack.

svgr iteself depends on these so it gets upgraded too.

Test Plan:

npm run start

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8814