Add contests↔︎sponsors check to validate script

[natevw]

There are some various checks in bin/validate.js that happen as part of the CI here:

    await validateHandles();
    await validateTeams();
    await validateOrganizations();
    await validateFindings();

But one other one that could be helpful is to validate the contests vs. sponsor data to flag any mismatch there. For example if a contest has been added in the main branch, a stale side branch can start getting a sort of cryptic error like:

2:26:22 PM: error Building static HTML failed for path "/contests/2023-03-aragon-protocol-contest"
2:26:22 PM: 
2:26:22 PM:   182 |               <div className="contest-page__project">
2:26:22 PM:   183 |                 <div className="contest-page__project-link project-link">
2:26:22 PM: > 184 |                   <a href={sponsor.link}>
2:26:22 PM:       |                                    ^
2:26:22 PM:   185 |                     <img
2:26:22 PM:   186 |                       src={sponsor.image.childImageSharp.resize.src}
2:26:22 PM:   187 |                       alt={sponsor.name}
2:26:22 PM: 
2:26:22 PM:   WebpackError: TypeError: Cannot read properties of null (reading 'link')

which isn't obvious but basically means the API is returning a contest but the dev branch is missing the sponsor info and avatar. The validate could detect this more directly and flag it in a clearer way to make it easier to troubleshoot.

[Noting that the bigger picture fix might be to keep the two data sources from getting out of sync in the first place, but that might be a longer-term effort.]

[natevw]

cf. #6203 (comment)