In-app vault export support (#729)

* First pass at vault export UI

* Password validation via cryptoService

* Export service framework

* support for constructing json export data

* Support for constructing csv export data

* Cleanup and simplification

* Completion of vault export feature

* Formatting and simplification

* Use dialog instead of toast for invalid master password entry

src/iOS.Core/Services/DeviceActionService.cs

@@ -133,6 +133,12 @@ namespace Bit.iOS.Core.Services
             return true;
         }
 
+        public bool SaveFile(byte[] fileData, string id, string fileName, string contentUri)
+        {
+            // OpenFile behavior is appropriate here as iOS prompts to save file
+            return OpenFile(fileData, id, fileName);
+        }
+
         public async Task ClearCacheAsync()
         {
             var url = new NSUrl(GetTempPath());