csp: support enketo-passthrough URLs in <iframe>

[alxndrsn]

Blocked by

• check-docker-context: fix "No such image" error #1649

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Closes #1637

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

• manually tested on dev.getodk.cloud
• updated tests in this repo

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

Fixes the issue simply.

#1533 should probably be followed up if more protection is wanted.

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 noticeable 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

[lognaturel]

Thank you!

[lognaturel]

#1533 should probably be followed up if more protection is wanted.

Very good point, agreed.