[Cashtab] formatDate function patch

Summary: Refactored format date to accept an optional useLocale paramater, included this parameter where the function is used throughout the app, and updated the unit tests to include a locale to ensure consistent results. Related to task T2139

Test Plan:
cd web/cashtab && npm start
Check that dates on wallet tab match browser's locale
Check that dates on SendToken page match browser's locale
In browser settings, switch to another language.
Check that dates on wallet tab match browser's locale
Check that dates on SendToken pafe match browser's locale.

npm test
ensure that all tests pass.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10791