nginx: reject form previews with unexpected query params

[alxndrsn]

What has been done to verify that this works as intended?

New tests, existing tests.

Why is this the best possible solution? Were any other approaches considered?

$args matching seems like a blunt implement, but nginx config is also quite limiting.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

No expected effect.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced