hide bottom-bar in subsequent views

before, this was randomly shown for some views and hidden for others.
i think it is better to hide it for all as otherwise,
the user may click on 'settings' and get the view he opened before
which might be a bit irritating.
fixes #324

deltachat-ios/Controller/EditSettingsController.swift

@@ -40,6 +40,7 @@ class EditSettingsController: UITableViewController {
     init(dcContext: DcContext) {
         self.dcContext = dcContext
         super.init(style: .grouped)
+        hidesBottomBarWhenPushed = true
     }
 
     required init?(coder aDecoder: NSCoder) {

deltachat-ios/Controller/MailboxViewController.swift

@@ -5,7 +5,7 @@ class MailboxViewController: ChatViewController {
 
     override init(dcContext: DcContext, chatId: Int) {
         super.init(dcContext: dcContext, chatId: chatId)
-        hidesBottomBarWhenPushed = false
+        hidesBottomBarWhenPushed = true
         disableWriting = true
         showCustomNavBar = false
     }

deltachat-ios/Controller/SettingsClassicViewController.swift

@@ -20,6 +20,7 @@ class SettingsClassicViewController: UITableViewController {
         self.options = [Int(DC_SHOW_EMAILS_OFF), Int(DC_SHOW_EMAILS_ACCEPTED_CONTACTS), Int(DC_SHOW_EMAILS_ALL)]
         super.init(style: .grouped)
         self.title = String.localized("pref_show_emails")
+        hidesBottomBarWhenPushed = true
     }
 
     required init?(coder aDecoder: NSCoder) {