[Cashtab] Use locale formatted numbers for agora offer previews

Summary: For large token values, it is difficult to read without locale thousands separators. Use already-available function to add them.

Test Plan:
npm test, npm start and view large offer to confirm. The function itself is already tested and should have been implemented here originally.

before:

after:

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17420