Add support for processing and playing audio only files

[jduehring]

This PR is a draft to add out-of-the-box audio-only support to Opencast. It adds conditions to the partial-publish WF, so that steps like timelinepreviews, theming etc. will be skipped when only an audio stream is detected. I also adds a basic encoding profile to encode the incoming audio stream to mp3. There are different ways to achieve actually playing the processed audiofile which I would like to propose / discuss:

1. Unloading Paellaplayer and loading the native HTML Audioplayer after detecting audio-only. This approach is currently used in this PR as it was my first approach.

2. After adding the "audio" streamtype to EpisodeConversor.json PaellaPlayer7 is actually capable of detecting/playing audio-only and by default displays the PaellaPlayer7 Logo:

3. In the past I often used a custom encoding profile to encode a video with a default background image from the audio-source. But since this ends up in a mp4 video I guess it should not be the standard solution, but rather playing actual audio-only files.

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[miesgre]

Keep in mind that option 1 completely disables paella and uses a standard player.
This has some implications. Do you want to show any of the paella plugins? For example, display event metadata? change the playbackrate?
Will subtitles of the event be generated? If subtitles are generated, with option 1 we would not see them and we should choose option 2.

Taking this into account, I would bet on option 2, or maybe configurable in config.json?
A poster frame could be generated and paella will show that image instead of the paella logo.
Example: https://paellaplayer.upv.es/#/demos/audio-and-images

[snoesberger]

I agree with @miesgre. From a user's point of view, I would prefer option 2. That way, the user always has the same control options, regardless of whether the stream is a video or audio only. For example, they can adjust the playback speed, show subtitles, skip forward or backward, etc., for audio-only streams, just as they are used to doing with video.

Option 1 is also likely to be browser dependent and the behaviour may vary between browsers.

[karendolan]

I also agree with @miesgre and @snoesberger about maintaining a similar experience for the student and maintaining the Paella player context for the published material. Perhaps, make the option to use the native player a configuration option.

[jduehring]

Thanks for your feedback!

I implemented a config option audioOnlyHTMLPlayer that will let you decide if you want to use option 1 or option 2.
If option 2 is chosen, a generated cover-image will be used a a preview picture and background in the PaellaPlayer. The cover-image uses the default coverimage.xsl but can of course be modified.