Don't show option to private chat with oneself

While this option works, it doesn't make much sense since there isn't
much of a good reason to talk to oneself aside from for testing
purposes. The created conversation also has confusing UI, showing as an
"empty room", not a DM. If needed, creating an empty room with E2E
encryption has the same effect as creating a private chat with
yourself.

Closes #446.

imports/NeoChat/Dialog/UserDetailDialog.qml

@@ -142,6 +142,8 @@ Kirigami.OverlaySheet {
             }
         }
         Kirigami.BasicListItem {
+            visible: user !== room.localUser
+
             action: Kirigami.Action {
                 text: i18n("Open a private chat")
                 icon.name: "document-send"