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: https://github.com/openedx/modular-learning/issues/191

Private ref: FAL-3655

Note: needs a version bump when merged.

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