Convert remaining issue board components into ES module syntax

EE port: gitlab-ee!7842

What does this MR do?

Removes the use of window.gl.issueBoards.* globals to manage the issue boards components.

What are the relevant issue numbers?

Part of: gitlab-org/gitlab-ce#44224 Related to: gitlab-org/gitlab-ce#34371

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides