Shorten sentence videos

[Jetske]

@uklomp after thinking of a doable way to implement video cropping functionality, I'd like to give it a try anyway. No promises though.

[Woseseltops]

For completeness, this is 'temporal cropping', that is 'trimming' or just 'cutting'

[Jetske]

@uklomp FYI this seems to be working! We need to test it on signbank-dev though. But first I'll add a way to add sources because that will be in the same update.

[susanodd]

@uklomp FYI this seems to be working! We need to test it on signbank-dev though. But first I'll add a way to add sources because that will be in the same update.

Can we put the batch edit on signbank yet? It's on signbank-dev. And the "moving keywords to different senses" is on signbank-susan. Those are both pull requests.