Don't let non-form-owner add voters

[ineiti]

Also adds a test in the script directory to make sure this doesn't pass anymore.

Closes #81

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[PascalinDe]

LGTM - there is a second admin in the Docker environment I added for manually testing things in my local instance (which is not needed for the automatic tests)

I didn't add it to the local environment setup as it is not required for anything other than my convenience, shall I add it anywayy?

[ineiti]

I didn't add it to the local environment setup as it is not required for anything other than my convenience, shall I add it anywayy?

Let's wait if we have other tests with two admins.

[ineiti]

Once #116 is merged, I rebase on this, and then hopefully it passes...