Coach view: hide dropdown when there are no practice quizzes

[sairina]

Summary

Frame	Description of expected behavior
	Coach user has access to channels with folders that have practice quizzes (channel quizzes) - dropdown menu with option to select a practice quiz is visible
	Coach user does not have access to channels that have folders with practice quizzes - normal KRouterLink to create a new quiz is visible

References

Addresses #8632

Reviewer guidance

For manual testing:

Frame 1: Coach user with practice quizzes on device

1. Sign in as super admin
2. Make sure there is a channel with a practice quiz on the device
3. Go to Coach in the sidenav
4. In Coach, click on Plan tab > Quizzes subtab
5. Check to see that the "New quiz" button is a dropdown (has a triangle) and that "Select a channel quiz" is an option (for the moment, it is "channel quiz" but when the strings are merged, this will become "practice quiz")

Frame 2: Coach user with NO practice quizzes in any channel on device

1. Sign in as super admin
2. Make sure there are NO channels with practice quizzes on the device
3. Go to Coach in the sidenav
4. In Coach, click on Plan tab > Quizzes subtab
5. Check to see that the "New quiz" button is not a dropdown (without a triangle) and that you are directed to creating a new quiz when you click on this button

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Testing checklist

• 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

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')

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

[rtibbles]

Functionality works perfectly (although I had some strange issues with the query not working properly at first until I reimported content, I am assuming it had to do with weird local DB state of mine).

Just needs to rebase the relevant commits on top of 0.15!