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/Services/ISendStorageService.cs

@@ -11,5 +11,6 @@ namespace Bit.Core.Services
         Task DeleteFileAsync(string fileId);
         Task DeleteFilesForOrganizationAsync(Guid organizationId);
         Task DeleteFilesForUserAsync(Guid userId);
+        Task<string> GetSendFileDownloadUrlAsync(string fileId);
     }
 }