Add type="button" to the accordion toggler

[fritzmg]

If you happen to insert an accordion content element in a form, e.g. in an HTML code form field with the {{insert_content::*}} insert tag, the toggler of the accordion will currently submit the form when clicked, instead of just opening/closing the accordion.

This is because any <button> without a specific type will be treated as a type="submit" button by default: https://html.spec.whatwg.org/multipage/form-elements.html#the-button-element

The attribute's missing value default and invalid value default are both the Submit Button state.

Since our toggler button is not supposed to submit anything but rather just act "as a button", it needs type="button".

[leofeyer]

Thank you @fritzmg.