Refactor students report in order to support environments with separated user table

[renatho]

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Steps to Reproduce

1. Use an environment where the user table is separated from the comments table.
2. Go to WP Admin > Sensei LMS > Reports.

What I Expected

To see the reports.

What Happened Instead

It doesn't list any student.

PHP / WordPress / Sensei LMS version

PHP 7.4 / WP 6.0 (multisite) / Sensei 4.5.1

Browser / OS version

Chrome

Context / Source

Sensei_Reports_Overview_Data_Provider_Students::get_items makes a query in the user table, but adds some actions to relate it to the comments table. Could we make a refactor to extract this relation from the query, and maybe join it through PHP?

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