pass course version name to batch certificate page - #48126

Finishes PLAT-1533. See #48000 and #48101 for context.

This PR does two user-visible things. when navigating to the batch certificates page from teacher home page, the batch certificate page will now show:

1. a customized certificate for that course
2. the display name of the course, e.g. "A printable page with personalized AI for Oceans certificates will be generated."

Everything user-facing in this PR is still hidden behind an experiment flag.

There are also these behind-the-scenes changes:
3. CurriculumHelper.find_matching_course_version now uses get_from_cache methods to avoid hitting the database
4. /certificate_images/... endpoint can now write any valid course name onto a blank certificate, not just csf (3adfd02)

Screenshots

Custom certificates for AI for Oceans:

Custom certificates for CSP:

Testing story

• I tested (1) manually. there is an existing eyes test which shows that the /certificates/batch page can show custom certificates, but that doesn't quite test this functionality end-to-end
• (2) is covered by new UI tests
• added unit tests for (3) and (4)

Follow-up work

In the CSP screenshot, you can see the course name is written as csp-2021, instead of "Computer Science Principles ('22-'23)" This is tracked by PLAT-1888 and is up next (#48145).