Fix: ck-button should ignore default -webkit-appearance.

[dkonopka]

Suggested merge commit message (convention)

Fix: ck-button should ignore default -webkit-appearance. Closes ckeditor/ckeditor5#3414.

---

Additional information

Various CSS frameworks (e.g. bootstrap) is styling button-like elements, that's why we need to reset them:

button,
html [type="button"], 
[type="reset"],
[type="submit"] {
  -webkit-appearance: button;
}