Fix issues

src/data/new_conversation.rs

@@ -2,8 +2,10 @@
 //!
 //! @author Pierre Huber
 
-use crate::data::user::UserID;
+use std::collections::HashSet;
+
 use crate::data::group_id::GroupID;
+use crate::data::user::UserID;
 
 #[derive(Debug)]
 pub struct NewConversation {
@@ -13,6 +15,6 @@ pub struct NewConversation {
     pub color: Option<String>,
     pub logo: Option<String>,
     pub owner_following: bool,
-    pub members: Vec<UserID>,
-    pub can_everyone_add_members: bool
+    pub members: HashSet<UserID>,
+    pub can_everyone_add_members: bool,
 }