Fix changelog form.
Add easy changelog form checking.
Add simple cleanup function.

To be edited~
Update error codes.
Update form checking.
Add error throwing function.
Add method comments, remove one global var.
Shorten the form validation.
Add a note about required MySQLI "link" argument.
Move query method to main function file.

That's because of required "link" argument.
Add change type param and working query.