Add lesson status icon for "complete" status

[donnapep]

Resolves #7594.

Proposed Changes

Adds an icon for when the lesson status is complete. I'm not sure how a lesson can arrive in this state as per this comment, but I suspect it has something to do with HPPS since we updated statuses as part of that project.

Testing Instructions

It's unclear how to test this without manipulating the database, but the lesson status icons in the sidebar of Learning Mode should all display correctly.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Legacy courses (course without blocks) are tested
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[github-actions]

Test the previous changes of this PR with WordPress Playground.