MM-38093: updates notifications copy on CRT replies (#18332)

MM-38093: updates notifications copy on CRT replies

Updates texts form push and email notifications when the user has CRT "on"
and the post is a reply to a thread.

Email batches text aware of CRT threads

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/user.go

@@ -38,6 +38,7 @@ const (
 	CommentsNotifyNever            = "never"
 	CommentsNotifyRoot             = "root"
 	CommentsNotifyAny              = "any"
+	CommentsNotifyCRT              = "crt"
 	FirstNameNotifyProp            = "first_name"
 	AutoResponderActiveNotifyProp  = "auto_responder_active"
 	AutoResponderMessageNotifyProp = "auto_responder_message"