The HTML5 video element already has a well-supported "playbackRate" attribute
which can be used to increase playback rate. This change wires up that control
to be displayed in the Jellyfish web player.
The playback rates offered are between 0.5x and 2x in 0.25x increments, which
matches the YouTube player. This change also wires up the ">" and "<" key
events to increase and decrease the playback rate, which mirrors the keyboard
shortcuts supported by YouTube.
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