Customize ContentUnavailablePage for different roles and change its URL

[kmrinal19]

Summary

This pull request incorporates following changes:

• Show learnerText message only if the user has the Learner role (and does not have the "can manage content role").
• Fix punctuation in the import channels link text for admin.
• Change URL of the ContentUnavailablePage from content-unavailable to resources-unavailable.

http://localhost:8000/en/learn/#/resources-unavailable when viewed from admin account

http://localhost:8000/en/learn/#/resources-unavailable when viewed from learner account

References

Fixes #7694

Contributor Checklist

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• PR has the correct target branch and milestone
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Testing:

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

Reviewer Checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[jonboiser]

Looks good, thanks!