Add context menu for follow notifications

Tusker/Views/Notifications/FollowNotificationGroupTableViewCell.swift

@@ -130,3 +130,20 @@ class FollowNotificationGroupTableViewCell: UITableViewCell {
     }
     
 }
+
+extension FollowNotificationGroupTableViewCell: MenuPreviewProvider {
+    
+    func getPreviewProviders(for location: CGPoint, sourceViewController: UIViewController) -> PreviewProviders? {
+        return (content: {
+            let accountIDs = self.group.notificationIDs.compactMap(MastodonCache.notification(for:)).map { $0.account.id }
+            if accountIDs.count == 1 {
+                return ProfileTableViewController(accountID: accountIDs.first!)
+            } else {
+                return AccountListTableViewController(accountIDs: accountIDs)
+            }
+        }, actions: {
+            return self.actionsForNotificationGroup(self.group)
+        })
+    }
+    
+}