fix(forum): forget the paths of the sections between requests
ForumSectionPathService kept the paths it had worked out for the life of the process. The functional suite boots the container once, and on SQLite a rolled back section hands its id to the next one, so a test could read the path of a section of another test: VisitorPlacesTest failed whenever the order put it after one. The service is reset before every request now, like the other request-scoped services.