Encrypt celery additional data

[kernicPanel]

Purpose

When analyzing sentry logs, we captured some issues:

• non relevant errors sent during newsletter subscription
• sensitive data sent (celery task arguments)

Proposal

As those sensitive data were crucial for debugging a real problem, all contexts are encrypted. An admin page is added to decrypt them

• [x] use logger.info on first call to brevo when subscribing user
• [x] encrypt sentry additional data
• [x] add an admin page to decrypt sentry context