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/App/Utilities/UpperCaseConverter.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Globalization;
+using Xamarin.Forms;
+
+namespace Bit.App.Utilities
+{
+    public class UpperCaseConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if(targetType != typeof(string))
+            {
+                throw new InvalidOperationException("The target must be a string.");
+            }
+
+            if(value == null)
+            {
+                return string.Empty;
+            }
+
+            return parameter.ToString().ToUpper();
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new NotSupportedException();
+        }
+    }
+}