Let `ds-link` be able to style `<button>`

[Barsnes]

A case for this is where you want a <form> to log out the user:

<form action="/logout" method="POST">
  <input type="hidden" name="${_csrf.parameterName}" value="${_csrf.token}">
  <button class="ds-link" type="submit">Logg ut</button>
</form>

Currently ds-link does not turn off the native <button> styling properly: