Subscription settings sample for email has validation errors and missing data-css-strict

## 🐛 Bug Report

### Current behavior

The "Subscription settings" sample for the email format currently has validation errors. Beside ones that can be trivially fixed, it uses a Mustache variable in the middle of an `amp-img` URL which is banned in the email spec. If Mustache variable is used, then it must be substituting the entire URL and not partially.

It's also missing the `data-css-strict` attribute on ``, which is required to enable strict CSS validation for email. After it's enabled, there's 1 CSS validation failure caused by using `cursor: progress`.

Please also provide:
- Page link: https://playground.amp.dev/?runtime=amp4email&url=https%3A%2F%2Fpreview.amp.dev%2Fdocumentation%2Fexamples%2Finteractivity-dynamic-content%2Fsubscription_settings

### Expected behavior/code
No validation error