Normalize button::moz-inner-focus for Firefox

[hbharding]

Originally mentioned in #2365.

While tabbing through components, Firefox shows a grey dotted border for some component's :focus state. This appears to only happen when hitting tab. Manually setting :focus in dev tools doesn't seem to trigger it. I'm not 100% sure, but from limited testing I think this focus style only applies to <button> elements.

If we decide to, we could normalize this behavior by doing something like:

# normalize.scss
button::moz-inner-focus {
  border: 0
}

[myasonik]

I had actually just gone ahead and done it in #2193 and no one said boo. But it was reverted due to some upgrade issues in Kibana...

It does seem like EUI's intent is to remove the default browser focus and provide our own custom one so I'm on board with removing it.

I think the property is ::-moz-focus-inner not ::moz-inner-focus though. When trying to confirm it, I also found :moz-focusring which seems to be their currently preferred API. (Couldn't find any official docs for either of the other options.)

Codepen showing *:-moz-focusring { outline: none !important } works