Add total descendant count to tag list APIs [FC-0036]

[bradenmacdonald]

This PR updates the APIs for retrieving the tags in the taxonomy, so that they state not only how many direct children they have, but also how many total descendants.

The direct children count is still included, not only to preserve backwards compatibility, but because it's necessary to compute pagination and display a "load more children" button when necessary.

What about performance of these queries? I ran the whole test suite before and after this change, which includes some very large taxonomies, and there was no measurable difference on average.

Ref: openedx/modular-learning#191

Private ref: FAL-3655

Note: needs a version bump when merged.

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[pomegranited]

👍 with one suggested change.

Once you address that and do a version bump, I think this is good to merge.

• I tested this on my devstack using openedx/frontend-app-authoring#826
  • Navigated to the taxonomies list and selected a taxonomy
  • Ensured that the descendant counts are showing as expected next to each tag in the tree.
• I read through the code
• I checked for accessibility issues by using my keyboard to navigate
• Includes documentation
• User-facing strings are extracted for translation N/A

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