[batch] Expose job group structure in the UI

[daniel-goldstein]

Fixes #14598

[daniel-goldstein]

There's more work to be done here. This adds a new route to the batch UI for getting a certain job group within a batch. It then, instead of listing all jobs, only lists the jobs that belong directly to the currently viewed job group and also shows the child job groups of the current job group.

When picking up this PR I would make sure to go through the Batch development tutorial to make sure you are familiar with dev deploying. Then, read this to learn about all the ways you can avoid dev deploying 😄

If you are only making tweaks in the HTML templates, you don't need to keep deploying for every little change. Instead, run

make devserver SERVICE=batch

in your terminal and you'll get a local server that proxies the Batch that your hail installation is pointed to. You can then make changes to HTML and refresh your browser to see the results. Note that this is just rendering the HTML locally, and will have any effect on what's deployed, meaning you can't use it for python changes.