Validate `resource_type` param

What does this MR do and why?

• Render a 404 if invalid class name is passed
• This endpoint is only available to instance admins but it is a best security practice not to call constantize
• http://gavinmiller.io/2016/the-safesty-way-to-constantize/

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.