Remove sessions after migration

[fflorent]

Problem

1. Install Garradin
2. Through a browser, initialize and ensure to be logged in Garradin
3. Run the migration to paheko
4. wait a little while until the post-migration script is run (check the output of at -l, it should be empty after 3 minutes)
5. Go back to the browser, an error should occur

Solution

• remove the session initialized by Garradin (all the users get logged out)

PR Status

• [x] Code finished and ready to be reviewed/tested
• [x] 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)

[fflorent]

NB: this branch has been successfully run locally and on production!