fix all members query in admin section

[hugosolar]

Fixes

Fixes creativecommons/tech-support#647 by @Juliabrungs

Description

This PR fixes the query for all users in the admin section. this happened because the prior query compare the users in the "Final Approval" form and there were some users that don't appear on that form because at the beginning of times the site didn't have that approval workflow.
I replace those users approval dates with the beginning date of the website (2017-10-15)

Technical details

I replace the query to the final approval form with the WP_User_Query WordPress class so we ensure all users are included now.

Checklist

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• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[zackkrida]

Looks good!

[zackkrida]

is this variable being used anywhere?

[hugosolar]

forgot to remove it. I'll do it immediately
thanks!