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/Core/Models/Export/Field.cs

@@ -0,0 +1,34 @@
+using Bit.Core.Enums;
+using Bit.Core.Models.View;
+
+namespace Bit.Core.Models.Export
+{
+    public class Field
+    {
+        public Field() { }
+
+        public Field(FieldView obj)
+        {
+            Name = obj.Name;
+            Value = obj.Value;
+            Type = obj.Type;
+        }
+
+        public string Name { get; set; }
+        public string Value { get; set; }
+        public FieldType Type { get; set; }
+
+        public static FieldView ToView(Field req, FieldView view = null)
+        {
+            if(view == null)
+            {
+                view = new FieldView();
+            }
+
+            view.Type = req.Type;
+            view.Value = req.Value;
+            view.Name = req.Name;
+            return view;
+        }
+    }
+}