Style: add a scrubber to the audio playback button; don't display any audio controls until the new one is rendered

[nonprofittechy]

See #29 for longer discussion.

I think we have just two fixes left to make:

1. Add a scrubber/way to reposition the audio playback. We discussed having this be something that only shows when the screen is being read aloud. It would display/collapse like an accordion.
2. Right now the old audio UI shows for a few seconds before the new one is rendered. It would be nice to have the old audio control start out hidden.

[BryceStevenWilley]

2 is a bit difficult, we're already hiding the old UI as soon as we can, right after a daPageLoad. We'd have to change DA core to have the audio controls start out as hidden. for it to be completely hidden the whole time and not pop up.