[fix] Create datadir after user is created

[yalh76]

Problem

• Fix #164
• Vaultwarden can't write to DATADIR
• can't find .env even if specified in the systemd unit.
• During upgrade, on migration from bitwarden, user vaultwarden does not exist when we set permission for the datadir

  2022-01-24 13:45:18,612: DEBUG - + chown -R vaultwarden:vaultwarden /home/yunohost.app/vaultwarden
  2022-01-24 13:45:18,616: WARNING - chown: invalid user: ‘vaultwarden:vaultwarden’

• Fix #166

Solution

• Add DATADIR in ReadWriteDirectories
• Rename vaultwarden.env to .env. We don't delete vaultwarden.env in case someone had some custom values.
• Create datadir after user is created

PR Status

• [ ] Code finished and ready to be reviewed/tested
• [ ] The fix/enhancement were manually tested (if applicable)

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[yalh76]

!testme

[yunohost-bot]

:rocket:

[Tagadda]

Maybe this need a ~ynh version bump

[yalh76]

Maybe this need a ~ynh version bump

Done at #170, and a big thank you for your help