Ahelp and Mhelp QOL (#24597)

* Bound ahelp/mhelp replies more tightly to their ticket.

* Hardened PMs further against missing data.

* Support expanding Discord notifications for ahelp/mhelp.

Functionality changes:
* Informs the user when their ticket is reopened.
* Informs other mentors/admins when tickets are converted.
* Discord messages now get a "Ticket 3, " or similar prefix.
  Not useful by itself, but will be with the following PR.

Code changes:
* When you create a ticket, you get the ticket object back.
* checkForTicket now allows you to fetch a ticket by number, and falls
  back to searching for open tickets if the ticket isn't found.
* Corrected and simplified the logic for adding PMs to tickets.
* Responses are now added through the ticket system, not the tickets
  directly.

code/modules/admin/misc_admin_procs.dm

@@ -44,7 +44,7 @@ GLOBAL_VAR_INIT(nologevent, 0)
 	for(var/client/C in GLOB.admins)
 		if(check_rights(R_ADMIN | R_MENTOR | R_MOD, 0, C.mob))
 			if(important || (C.prefs && !(C.prefs.toggles & PREFTOGGLE_CHAT_NO_TICKETLOGS)))
-				to_chat(C, msg, MESSAGE_TYPE_MENTORCHAT, confidential = TRUE)
+				to_chat(C, msg, MESSAGE_TYPE_MENTORPM, confidential = TRUE)
 			if(important)
 				if(C.prefs?.sound & SOUND_MENTORHELP)
 					SEND_SOUND(C, sound('sound/effects/adminhelp.ogg'))