Added stub for "get_all_users" user service.

[ShawnMilo]

This is for SGA issue 115:

issue 115

Presented here for comment.

[openedx-webhooks]

Thanks for the pull request, @ShawnMilo! I've created OSPR-780 to keep track of it in JIRA. JIRA is a place for product owners to prioritize feature reviews by the engineering development teams.

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[pdpinch]

@ShawnMilo works for MIT ODL and is covered by the MIT agreement.

[ShawnMilo]

I meant to do a PR to the mitodl fork. Shall we delete this one?