ios: UI to export/import/delete chat database (#743)

* ios: UI to export/import/delete chat database

* move files

* ui for database migration

* migration screen layout

* ios: export archive and delete chat database

* import archive

* refactor, update texts

* database migration (almost works)

* fix missing import

* delete legacy database

* update migration errors

apps/ios/Shared/Model/ChatModel.swift

@@ -15,6 +15,8 @@ import SimpleXChat
 final class ChatModel: ObservableObject {
     @Published var onboardingStage: OnboardingStage?
     @Published var currentUser: User?
+    @Published var chatRunning: Bool?
+    @Published var chatDbChanged = false
     // list of chat "previews"
     @Published var chats: [Chat] = []
     // current chat