Stricter types

[victorlin]

Mirrors https://github.com/nextstrain/nextstrain.org/pull/1077

Checklist

• [x] Checks pass
• [x] If making user-facing changes, add a message in CHANGELOG.md summarizing the changes in this PR
• [x] (to be done by a Nextstrain team member) Create preview PRs on downstream repositories.

[victorlin]

Merging pre-review since https://github.com/nextstrain/nextstrain.org/pull/1077 was already approved.