Secret comunity

This MR adds a new privacy option for communities. The new type is "secret" and this concept is different from banned communities(that should block every kind of action on the community). With this option the community won't be found anywhere(search and blocks). If someone try to access it by URL it gives access_denied and you can't ask to join this community, you can only get in if someone inside the community invites you. The only way a member can see the community is by the "Manage my Groups" page. With the secret status, a community can't create public contents unlike private communities.