Moved CSS for transcript button into core mediaelement stylesheet (Bug #1264)

* From, https://github.com/IET-OU/mediaelement-demo/
* https://github.com/IET-OU/mediaelement/blob/iet-ou/CR1264/a11y-transcript/src/css/
* https://github.com/johndyer/mediaelement/issues/1264