tweak sessions end email

-   appointments are an optional feature, so we don't care whether they
    are correctly maintained
-   participations must be correctly maintained, otherwise it is
    impossible to e.g. access pseudonyms. SUggesting that users could
    let this slide and enter participants at the end of the study just
    incourages incorrect and potentially harmful usage.
-   Finishing a study and deleting its pseudonym lists is crucial for GDPR
    compliance because it removes the study as a legal basis.