Cached department and topic page counts

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/5564

Description (What does it do?)

This PR updates the /topics and /departments channel pages to use the new cached counts api endpoints to fetch program and course counts.

How can this be tested?

1. visit the production MIT Learn topics and departments channel pages with web inspector open. note that the page makes multiple requests to fetch counts to the search endpoint (looks something like /api/v1/learning_resources_search/?aggregations=department&resource_type=course)
2. checkout this branch. make sure you have topics and departments loaded up locally.
3. visit the topics and departments channel pages and note that the requests to the search endpoint do not show up. Also verify the counts for programs and courses look accurate. note: the counts endpoint is cached so it wont update immediately after deleting something - you will need to clear the cache manually to see it update

[abeglova]

lgtm