Fix some reports not exporting all rows

[donnapep]

Resolves #7666.

Proposed Changes

• Reverts Remove limit from fetching data for data export #7647.
• Conditionally sets the number arg based on which report is being exported:
  • Sets it to an empty string for the students report, as that ultimately ends up getting passed to the get_comments function inside sensei_check_for_activity. As per this doc, an empty string should be used to return all rows.
  • Sets it to -1 for the lessons report, as that is ultimately used to set posts_per_page for WP_Query. As per this doc, -1 should be used to show all posts.

Testing Instructions

1. Go to Reports > Courses and select a course.
2. Export the rows (ensure there are multiple) and check that they were all exported.
3. Click on Students taking this Course.
4. Export the rows (ensure there are multiple) and check that they were all exported.
5. Go to Reports > Lessons and select a course.
6. Export the rows (ensure there are multiple) and check that they were all exported.

Note that I tested exporting all other reports and they work.

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.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[donnapep]

Merging as I've already done manual testing as well as added automated tests.