Improve client side script

[k1sul1]

Some (minor) changes that should make using WPLF for everything easier.

Tested on latest FIrefox, Safari & Chrome.

• Moved form submit function to the global wplf object so it can be used as a reference
  • You weren't able to easily remove anonymous event listeners, and if you removed it the hard way, good luck adding it back.
  • Now you can remove said listener like this: form.removeEventListener('submit', window.wplf.submitHandler), adding works the same way.
• Print a warning message to the console only if there's no errorCallback
• Fixed indent
• improved readability (and ruined diffs as a side effect)

[anttiviljami]

I'm getting reports this works fine. 👍

props Mika Miinalainen for testing this.

Thanks again, @k1sul1 !