Fix user mentions in Slack notifications (#1846)

Adding link_names parameter to ensure that incoming webhooks get properly parsed for user mentions. Without this user mentions remain as plain text.

src/Ombi.Api.Slack/SlackApi.cs

@@ -23,6 +23,7 @@ namespace Ombi.Api.Slack
             body.channel = message.channel;
             body.text = message.text;
             body.username = message.username;
+            body.link_names = 1;
 
             if (!string.IsNullOrEmpty(message.icon_url))
             {