Update email signup form iFrame url

[svetly]

We received notice from prefinery that we need to switch our signup form on gridstylesheets.org as follows:

We've recently made some changes to the internal URLs used to integrate with Prefinery. Normally this would be an invisible change; however, it looks as if your email signup forms were coded using an iframe directly pointing to a URL on our site instead of using our javascript widget code.

Our logs show that the form installed for Grid Style Sheets is still being seen and submitted by visitors. If you are still using this form, please update the URL before August 15 so that your email signup form continues to work. After that date visitors will see an embedded 404 page in that iframe.

The new iframe src URL for this form is: https://i.prefinery.com/projects/9w9vekgo/users/new?display=inline&version=2

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That said, prefinery hides our users and if we want access we need to pay 100/month. We should switch to a Mailchimp form going forward and migrate the subscriber list, currently we have 1800 hidden subscribers in prefinery.

Quick fix is to simply update the iFrame.