hide bottom bar in preview controller

preview controller may use it's own bottom bar,
so that's easily gets a ui mess.

(it was hidden before coincidentally as the parent's bottom bar was hidden)

deltachat-ios/Controller/PreviewController.swift

@@ -17,6 +17,7 @@ class PreviewController: QLPreviewController {
         self.previewType = type
         self.dcContext = dcContext
         super.init(nibName: nil, bundle: nil)
+        hidesBottomBarWhenPushed = true
         dataSource = self
         switch type {
         case .multi(_, let currentIndex):