Video Speed menu is not accessible

[swashbuck]

Subject of the issue

The _speed player option is not accessible:

1. The speed selector button does not have a meaningful label for screenreaders. The button is announced by default as the "one point zero zero x button"
2. The speed selector button is not keyboard accessible. Mouse users can hover over the playback speed button to reveal more options, but when a keyboard users sets focus on the playback speed button and activates it with space or enter, nothing happens. Keyboard users are unable to change their playback speed.

In mediaLibrariesOverrides.js, we override (or add?) aria labels for use with MediaElement.js. So, perhaps we could add a label for speed here.

The focus issue is similar to that of captions. See #232

It may be possible to fix this by updating mediaelement / mediaelement-plugins? See https://github.com/mediaelement/mediaelement-plugins/blob/42dc804df0e98ea8de4d07a736690b72585d9c37/src/speed/speed.js#L110

Steps to reproduce

Add 'speed' to a Media component's player options. For example:

"_playerOptions": {
  "alwaysShowControls": false,
  "hideVideoControlsOnLoad": true,
  "showPosterWhenEnded": true,
  "features": [
    "playpause",
    "progress",
    "fullscreen",
    "volume",
    "speed"
  ]
},

Then, navigate to the option with a screenreader / keyboard navigation.

[swashbuck]

Created #298 to look into updating the MediaElement library.

[swashbuck]

Needs reviewing as part of #299