fix(emails): Fix user's locale in emails from being overwritten by an other user

[xlisachan]

Because

• moment.tz.setDefault() and moment.locale() are global mutations
• Since Node.js processes emails concurrently (via async), one user's email could overwrite the global locale while another user's email was mid-render, causing that email to format dates with the wrong locale (e.g., MM/DD/YYYY instead of DD/MM/YYYY or vice versa)

This pull request

• Switches to instance-level .locale (locale) so each date formatting call is self-contained and can't affect others.

Issue that this pull request solves

Closes: PAY-3575

Checklist

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• If applicable, I have modified or added tests which pass locally.