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/Abstractions/IExportService.cs

@@ -0,0 +1,11 @@
+using System.Threading.Tasks;
+
+namespace Bit.Core.Abstractions
+{
+    public interface IExportService
+    {
+        Task<string> GetExport(string format = "csv");
+        Task<string> GetOrganizationExport(string organizationId, string format = "csv");
+        string GetFileName(string prefix = null, string extension = "csv");
+    }
+}