users: huge cleanup/dedupe, interface-based third-party services

shared "newUser" method is now "NewUserPostVerification", and is shared
between all routes which create a jellyfin account. The new
"NewUserFromInvite", "NewUserFromAdmin" and "NewUserFromConfirmationKey"
are smaller as a result. Discord, Telegram, and Matrix now implement the
"ContactMethodLinker" and "ContactMethodUser" interfaces, meaning code
is shared a lot between them in the NewUser methods, and the specifics
are now in their own files. Ombi/Jellyseerr similarly implement a
simpler interface "ThirdPartyService", which simply has ImportUser and
AddContactMethod routes. Note these new interface methods are only used
for user creation as of yet, but could likely be used in other places.

logmessages/logmessages.go

@@ -115,7 +115,7 @@ const (
 	FailedGetJellyseerrNotificationPrefs = "Failed to get user \"%s\"'s notification prefs from " + Jellyseerr + ": %v"
 	FailedSyncContactMethods             = "Failed to sync contact methods with %s: %v"
 	ImportJellyseerrUser                 = "Triggered import for " + Jellyseerr + " user \"%s\" (New ID: %d)"
-	FailedMustGetJellyseerrUser          = "Failed to get or trigger import for " + Jellyseerr + " user \"%s\": %v"
+	FailedImportUser                     = "Failed to get or trigger import for %s user \"%s\": %v"
 
 	// api-messages.go
 	FailedGetCustomMessage   = "Failed to get custom message \"%s\""