play: load relevant media player based on mime type

[Trek Hopton]

We have access to the mime type so we need to use this to choose which player to load. This will be either the MJPEG HLS player, or the H264 HLS (hls.js) player.

Initially the play page should only load the elements that both players need eg. URL input, and then load elements that are specific to the mime type dynamically.

[Trek Hopton]

Resolved in PR #54