Allow any sl-buttons to submit post requests

[benopotamus]

With the following code, if the user clicks the button, the form submits, because the button has a name and value attribute. See https://javascriptdev.info/how-to-make-an-html-button-perform-a-post-request/

<form action="" method="post">
    <button name="foo" value="upvote">Upvote</button>
</form>

This doesn't currently work with sl-buttons - only an sl-button with type='submit' will cause the form to submit.