Updating Firefox Accounts strings to Mozilla accounts

[reemhamz]

One-line summary

We're rebranding Firefox Accounts to Mozilla accounts (lowercase).

• Guidelines
• Spreadsheet

Testing

• Set Dev=False in .env
• Set SWITCH_MOZ_ACCOUNTS_UPDATE=True in .env

This is also running on a dev server: http://www-demo6.allizom.org/

Name	URL
Firefox accounts page	https://www.mozilla.org/en-US/firefox/accounts/
Email Preference Page	https://www.mozilla.org/en-US/newsletter/existing/?utm_source=pref_footer
Firefox Sync page	https://www.mozilla.org/en-US/firefox/sync/
Brave comparison page	https://www.mozilla.org/en-US/firefox/browsers/compare/brave/
Safari comparison page	https://www.mozilla.org/en-US/firefox/browsers/compare/safari/
Password manager page	https://www.mozilla.org/en-US/firefox/features/password-manager/
Relay FAQ page	https://www-dev.allizom.org/en-US/products/relay/faq/
VPN windows download page	https://www.mozilla.org/en-US/products/vpn/download/windows/thanks/
VPN mac download page	https://www.mozilla.org/en-US/products/vpn/download/mac/thanks/
Firefox Privacy Products page	https://www.mozilla.org/en-US/firefox/privacy/products/