[$40] Unauthenticated users should only search challenges without groups

[ThomasKranitsas]

Since the query fetches the first N challenges from ES, if the call is unauthenticated, we should add a filter so groups is undefined.

Currently, if the first 20 records belong to a private group, the GET /challenges will always return [] even if there are millions of challenges available.

[ThomasKranitsas]

@topcoder-platform/tcxcommunity @topcoder-platform/topcodercompetitors this is open for pickup. Please create a PR against develop branch.

[ThomasKranitsas]

Contest https://www.topcoder.com/challenges/30121722 has been created for this ticket.

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[ThomasKranitsas]

Contest https://www.topcoder.com/challenges/30121722 has been updated - it has been assigned to Narekvar90.

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[narekcat]

Hello @ThomasKranitsas . I finished this issue, see pull request #64