Enable "continue" in quiz creation only once exercises selected

[marcellamaki]

Summary

This adds a conditional check during quiz creation, and only allows a user to continue once one or more exercises have been selected.

References

Fixes #9472

continue-enabled-on-selection.mp4

Reviewer guidance

1. Go to create a new quiz
2. Continue button should be disabled at start
3. Continue button should be enabled after selecting at least one exercise
4. unselecting all exercises (individually or for example at the folder level) should re-disable the continue button

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')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

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

[radinamatic]

Continue button is now enabled only if at least one exercise has been selected 👍🏽
Tested on Firefox & Chrome on Ubuntu 18 and Windows 10.

[radinamatic]

Manual QA passed, good to go! 💯