reuse existing ephemeral icon

we already have an ephemeral icon,
that is also used across systems;
use that icon for the profile button.
(however, still do that only for ios>=13
as the other images are available only with ios>=13
and is it better to show no image at all for ios12
(this is no worsening as before we also had no icons at all))

deltachat-ios/Controller/ContactDetailViewController.swift

@@ -28,7 +28,7 @@ class ContactDetailViewController: UITableViewController {
         let cell = UITableViewCell(style: .value1, reuseIdentifier: nil)
         cell.textLabel?.text = String.localized("ephemeral_messages")
         if #available(iOS 13.0, *) {
-            cell.imageView?.image = UIImage(systemName: "timer") // added in ios13
+            cell.imageView?.image = UIImage(named: "ephemeral_timer")?.withTintColor(UIColor.systemBlue)
         }
         cell.accessoryType = .disclosureIndicator
         return cell

deltachat-ios/Controller/GroupChatDetailViewController.swift

@@ -82,7 +82,7 @@ class GroupChatDetailViewController: UIViewController {
         let cell = UITableViewCell(style: .value1, reuseIdentifier: nil)
         cell.textLabel?.text = String.localized("ephemeral_messages")
         if #available(iOS 13.0, *) {
-            cell.imageView?.image = UIImage(systemName: "timer") // added in ios13
+            cell.imageView?.image = UIImage(named: "ephemeral_timer")?.withTintColor(UIColor.systemBlue)
         }
         cell.accessoryType = .disclosureIndicator
         return cell