Make field names consistent

[mholt]

"challenge" vs. "challenge_type"... "identifier" vs. "identifiers"... etc.

For identifier/identifiers, we might need to make them all plural (arrays, even if only 1?) or for some of them perhaps users will just have to query like identifier = X OR contains(identifiers, X)... I dunno.

Using consistent field names make querying easier overall.

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