Ability to publish subset of requests from a private batch

[garethrees]

We've had this request a few times.

Giving more control over what's published from a batch can help pro users get more published sooner (rather than having to keep the entire batch private until all of the requests can be published), and avoids the use of private links being used to refer back to requests (these were intended more as a way to share in-progress requests; the URL isn't very human friendly).

This would add complexity to managing privacy periods across the entire batch after its creation, so we'd have to think about what the workflow is there when privacy periods are not identical for all requests in the batch.

See:

• https://groups.google.com/a/mysociety.org/g/alaveteli-pro/c/ZvHACB2mnmU/m/YPxDla3UEgAJ
• https://twitter.com/_gareth/status/1764960497273626775

[WilliamWDTK]

Just to note that we've had a WDTK Pro user get in touch recently, suggesting this:

So for instance, […] I'm happy for the batch to be put in the public domain apart from one or two requests from that batch I'm still working on/want to place a further embargo on them. At the moment, I can either publish all requests, and not individual requests. […] I'm sometimes forced to choose between publishing everything or not at all.