investigate a lighter-weight API for polling learner classroom status

[indirectlylit]

Observed behavior

In #5737 we added polling to refresh learners' classes.

Expected behavior

We should investigate performance impact of this polling, and if necessary create a dedicated, lighter-weight API for the polling endpoint, or find ways to optimize the existing endpoint.

User-facing consequences

potentially degraded performance

Steps to reproduce

Load test with many users polling simultaneously

Context

0.12.5

[rtibbles]

One possibility would be to add an etag or last modified to the endpoint if we could calculate this efficiently.