Groups - memberships - events - attendances need access control

[rabble]

If a user tries to view a group, memberships, events, or attendances of a group to which they shouldn't be able to manage they should be redirected somewhere else, either to the /root with a flash message if they aren't members, or to the public member version of the page if htey are a member but not an organizer.

[rabble]

You only got the controls for group and attendance but didn't do it for events or membership!

[matinieves]

@rabble Did you tested it? Were you able to access a page you should not?