Fix broken tests due to invalid field name

Description

`ContentLibraryPermission` refers to `ContentLibrary` with a `related_name` of `permission_grants`, which should be used in ManyRelation instead of the lowercase model name (see [docs](https://bridgekeeper.readthedocs.io/en/latest/api/rules.html#bridgekeeper.rules.ManyRelation))

*Testing instructions*:
This PR changes a blockstore based API, which is not part of the devstack yet. To test this:

  • From the blockstore directory, run `make testserver` to start a test-specific instance.

  • From `make studio-shell` run: `EDXAPP_RUN_BLOCKSTORE_TESTS=1 python -Wd -m pytest --ds=cms.envs.test openedx/core/djangoapps/content_libraries/tests/`

*Reviewers*

  • [ ] @bradenmacdonald

  • [ ] edX reviewer[s] TBD

Assignee

Kyle McCormick

Reporter

Open Source Pull Request Bot

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Sid Verma

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edx/edx-platform

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