MM-20681 Fix custom post types not marking channel unread when using Mark as Unread (#13247)

* MM-20681 Fix custom post types not marking channel unread when using Mark as Unread

* Fix inverted if statements

model/post.go

@@ -350,6 +350,19 @@ func (o *Post) IsSystemMessage() bool {
 	return len(o.Type) >= len(POST_SYSTEM_MESSAGE_PREFIX) && o.Type[:len(POST_SYSTEM_MESSAGE_PREFIX)] == POST_SYSTEM_MESSAGE_PREFIX
 }
 
+func (o *Post) IsJoinLeaveMessage() bool {
+	return o.Type == POST_JOIN_LEAVE ||
+		o.Type == POST_ADD_REMOVE ||
+		o.Type == POST_JOIN_CHANNEL ||
+		o.Type == POST_LEAVE_CHANNEL ||
+		o.Type == POST_JOIN_TEAM ||
+		o.Type == POST_LEAVE_TEAM ||
+		o.Type == POST_ADD_TO_CHANNEL ||
+		o.Type == POST_REMOVE_FROM_CHANNEL ||
+		o.Type == POST_ADD_TO_TEAM ||
+		o.Type == POST_REMOVE_FROM_TEAM
+}
+
 func (p *Post) Patch(patch *PostPatch) {
 	if patch.IsPinned != nil {
 		p.IsPinned = *patch.IsPinned