Questions and answers of affected sessions are now deleted.

Lists of visited sessions for guest users which have not been active for
a fixed amount of time are now automatically deleted.

The cleanup is now disabled by default and can be set up by uncommenting
the configuration parameter `session.guest-session.cleanup-days`.

This reverts commit 3e969181.

This reverts commit 5f5699c5.

The backend wrongly trusted user input allowing the circumvention of
access control checks.

Free text validation is no longer done for abstentions.

This fixes a regression introduced in
9fc360d6.