New mail services and implementations

src/Core/Models/Mail/BaseMailModel.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Bit.Core.Models.Mail
+{
+    public class BaseMailModel
+    {
+        public string SiteName { get; set; }
+        public string WebVaultUrl { get; set; }
+    }
+}

src/Core/Models/Mail/MailMessage.cs

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+using System.Collections.Generic;
+
+namespace Bit.Core.Models.Mail
+{
+    public class MailMessage
+    {
+        public string Subject { get; set; }
+        public IEnumerable<string> ToEmails { get; set; }
+        public string HtmlContent { get; set; }
+        public string TextContent { get; set; }
+        public IDictionary<string, object> MetaData { get; set; }
+    }
+}

src/Core/Models/Mail/MasterPasswordHintViewModel.cs

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+namespace Bit.Core.Models.Mail
+{
+    public class MasterPasswordHintViewModel : BaseMailModel
+    {
+        public string Hint { get; set; }
+    }
+}