FEATURE: add support for group members visibility level (#8004)

There are 5 visibility levels (similar to group visibility)

public (default)
logged-in users
members only
staff
owners

Admins & group owners always have visibility to group members.

spec/models/group_spec.rb

@@ -716,6 +716,71 @@ describe Group do
 
   end
 
+  describe ".members_visible_groups" do
+
+    def can_view?(user, group)
+      Group.members_visible_groups(user).exists?(id: group.id)
+    end
+
+    it 'correctly restricts group members visibility' do
+      group = Fabricate.build(:group, members_visibility_level: Group.visibility_levels[:owners])
+      logged_on_user = Fabricate(:user)
+      member = Fabricate(:user)
+      group.add(member)
+      group.save!
+
+      owner = Fabricate(:user)
+      group.add_owner(owner)
+
+      moderator = Fabricate(:user, moderator: true)
+      admin = Fabricate(:user, admin: true)
+
+      expect(can_view?(admin, group)).to eq(true)
+      expect(can_view?(owner, group)).to eq(true)
+      expect(can_view?(moderator, group)).to eq(false)
+      expect(can_view?(member, group)).to eq(false)
+      expect(can_view?(logged_on_user, group)).to eq(false)
+      expect(can_view?(nil, group)).to eq(false)
+
+      group.update_columns(members_visibility_level: Group.visibility_levels[:staff])
+
+      expect(can_view?(admin, group)).to eq(true)
+      expect(can_view?(owner, group)).to eq(true)
+      expect(can_view?(moderator, group)).to eq(true)
+      expect(can_view?(member, group)).to eq(false)
+      expect(can_view?(logged_on_user, group)).to eq(false)
+      expect(can_view?(nil, group)).to eq(false)
+
+      group.update_columns(members_visibility_level: Group.visibility_levels[:members])
+
+      expect(can_view?(admin, group)).to eq(true)
+      expect(can_view?(owner, group)).to eq(true)
+      expect(can_view?(moderator, group)).to eq(false)
+      expect(can_view?(member, group)).to eq(true)
+      expect(can_view?(logged_on_user, group)).to eq(false)
+      expect(can_view?(nil, group)).to eq(false)
+
+      group.update_columns(members_visibility_level: Group.visibility_levels[:public])
+
+      expect(can_view?(admin, group)).to eq(true)
+      expect(can_view?(owner, group)).to eq(true)
+      expect(can_view?(moderator, group)).to eq(true)
+      expect(can_view?(member, group)).to eq(true)
+      expect(can_view?(logged_on_user, group)).to eq(true)
+      expect(can_view?(nil, group)).to eq(true)
+
+      group.update_columns(members_visibility_level: Group.visibility_levels[:logged_on_users])
+
+      expect(can_view?(admin, group)).to eq(true)
+      expect(can_view?(owner, group)).to eq(true)
+      expect(can_view?(moderator, group)).to eq(true)
+      expect(can_view?(member, group)).to eq(true)
+      expect(can_view?(logged_on_user, group)).to eq(true)
+      expect(can_view?(nil, group)).to eq(false)
+    end
+
+  end
+
   describe '#remove' do
     before { group.add(user) }