Use sas token for send downloads (#1157)

* Remove Url from SendFileModel

Url is now generated on the fly with limited lifetime.

New model houses the download url generated

* Create API endpoint for getting Send file download url

* Generate limited-life Azure download urls

* Lint fix

src/Core/Models/Api/Response/SendFileDownloadDataResponseModel.cs

@@ -0,0 +1,10 @@
+namespace Bit.Core.Models.Api.Response
+{
+    public class SendFileDownloadDataResponseModel : ResponseModel
+    {
+        public string Id { get; set; }
+        public string Url { get; set; }
+
+        public SendFileDownloadDataResponseModel() : base("send-fileDownload") { }
+    }
+}