Levels and categories in channel details modal

[rtibbles]

Summary

Description of the change(s) you made

• Adds an aggregation helper class for label maps
• Uses it to return categories and levels for channel details
• Displays those channel details in the channel details modal - makes a slight deviation from the spec after consultation with @jtamiace to use chips instead of comma separation for consistency with other elements
• Cleans up ContentLevels constants, as they were being defined twice
• Removes unnecessary prefetch_related and select_related in node details method - seems to have no impact on performance either way

Manual verification steps performed

1. Add levels and categories to resources and topics in a channel
2. Verify that they appear, deduplicated in the channel details modal
3. Note that our caching may cause stale data to the be returned to the frontend, so the cache needs to be cleared

Screenshots (if applicable)

References

Fixes #3877

[marcellamaki]

This looks good and manual QA checks out for both the in-Studio display and PDF download.

My only (non-blocking) question is - can you remind me, did we decide to remove the parent category for category display? i.e. "Mathematics", though it also checks the "school" checkbox in the form, actually doesn't display the chip for "school" in the form, or in the details (at either the resource or channel level)

Just want to make sure this was an intentional change -- I think it was? -- and not a side effect of refactoring somewhere along the line.

[rtibbles]

Yes, pretty sure it was intentional to prevent display bloat.