design: Make not joined participants transparent in list

This should make it easier to distinguish
between actual participants of a
room and users who have been
invited, who are knocking or who
have been banned.

lib/pages/chat_details/participant_list_item.dart

@@ -25,7 +25,9 @@ class ParticipantListItem extends StatelessWidget {
             ? L10n.of(context).moderator
             : '';
 
-    return ListTile(
+    return Opacity(
+      opacity: user.membership == Membership.join ? 1 : 0.5,
+      child: ListTile(
         onTap: () => showModalBottomSheet(
           context: context,
           builder: (c) => UserBottomSheet(
@@ -56,12 +58,15 @@ class ParticipantListItem extends StatelessWidget {
                       color: Theme.of(context).secondaryHeaderColor,
                       borderRadius: BorderRadius.circular(8),
                     ),
-                  child: Center(child: Text(membershipBatch[user.membership])),
+                    child:
+                        Center(child: Text(membershipBatch[user.membership])),
                   ),
           ],
         ),
         subtitle: Text(user.id),
-      leading: Avatar(mxContent: user.avatarUrl, name: user.calcDisplayname()),
+        leading:
+            Avatar(mxContent: user.avatarUrl, name: user.calcDisplayname()),
+      ),
     );
   }
 }