[Email] Update welcome content (#1092)

* Initial commit of welcome email update

* Final revisions and updated hosted image urls

* added dynamic year // updated verbiage // fixed typo in text template

* Updated verbiage // changed mustache accessor // updated how year is generated

src/Core/Models/Mail/BaseMailModel.cs

@@ -18,5 +18,12 @@ namespace Bit.Core.Models.Mail
                 return WebVaultUrl;
             }
         }
+        public string CurrentYear
+        {
+            get
+            {
+                return DateTime.UtcNow.Year.ToString();
+            }
+        }
     }
 }