migration to debian bullseye

It mostly works, but there are remaining tasks before merging:

• should use the python3-josepy python3-jwt (from debian) instead of jose (from pypi)
• should find an alternative to pycrypto (no longer maintained)
• update the nginx TLS parameters
• test it in qualif, especially:
  • allauth integration
  • pushing docker images (through allgo.aio)