Add slide segments and extracted text to harvesting and DB, enable paella slide previews

[owi92]

This adds the ocr'd slide texts as well as a list of timestamped frames to the harvesting sync code and stores them in the DB.
In order the show the slide previews, paella-slide-plugins was added and configured to use the timestamped frames.

Needs opencast/opencast#5757 to work. Once that is merged, released and used on our test Opencast, the changes can be tested with fresh uploads. We'll still need some mechanism to apply segmentation and ocr (and speech-to-text as well) to existing videos.

(Can be reviewed commit by commit, though note that the migration from the second commit was extended in the third)

[LukasKalbertodt]

(Just code review, no testing yet)
Looks overall pretty good. I have a few small nits, but there is only one bigger one: about how to store the startTime.

[LukasKalbertodt]

Those are some high numbers... is that intentional?

[owi92]

Oh good call. I copied that from https://paellaplayer.upv.es/#/playground and forgot to adjust those numbers.

[LukasKalbertodt]

I think the tabIndex: 17 is still there? Or is that intentional now?

[github-actions]

This pull request has conflicts ☹
Please resolve those so we can review the pull request.
Thanks.

[LukasKalbertodt]

Quick note for everyone: due to how our test deployment works, existing videos will not have segments shown in our Tobira test deployments. Until the next Tobira release happens: then everything works as expected. Also: we can manually run a resync on the test deployment and sometimes we might do that, so it might work randomly. Don't let this confuse you :D