Update Parsec SaaS emails

[fabienSvtr]

• [x] Scille/parsec-bms#1056
  • [x] create an app with a sender to manage all email templates in the project
  • [x] update global base template (header & footer) according to mock-ups
  • [x] improve template format (html & css)
  • [x] update feedback email template according to mock-ups to work with the whole new email sending system
  • [x] add a send_email CLI command taking recipients, template name and email locale as params
  • [x] use django i18n (.po files) to translate email templates
  • [x] update documentation to reflect all the changes (env vars, new commands, new format...)
• [ ] Update email templates according to mock-ups: https://www.figma.com/design/jDUDnJmNiEwwy7UsAUI9cd/Email?m=auto&t=RzNzhIPVpBn34wUL-6
  • [ ] Invitation mail https://github.com/Scille/parsec-cloud/pull/8289
  • [ ] Scille/parsec-bms#1057
  • [ ] Scille/parsec-bms#1058
  • [ ] Scille/parsec-bms#1059
  • [ ] Scille/parsec-bms#1060
  • [ ] Scille/parsec-bms#1061
• [ ] Scille/parsec-bms#1062