Messages rights issue

We have multiple rights issue in the Messages tab:

• messages are not protected from editing by other people than group admin

So we need to:

• add an "edit" right assign to Group Admins on creating of the message
• add an "edit" right to the message creator

We need to check that (I think is ok already)

• the Messages space has "comment" right assigned to the members of the group if the group is closed

We need to hide the Edit/Comment/Attach button when people don't have the right