Add a "Thank You" page for MailChimp subscribers (#1642)

src/components/ThankYou/index.tsx

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+import React from 'react'
+import styles from './styles.module.css'
+
+const ThankYouPage: React.FC = () => (
+  <div className={styles.wrapper}>
+    <h1 className={styles.heading}>Thanks for subscribing!</h1>
+    <p className={styles.copy}>Keep an eye on your inbox</p>
+  </div>
+)
+
+export default ThankYouPage

src/components/ThankYou/styles.module.css

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+.wrapper {
+  margin: 12rem auto;
+  text-align: center;
+
+  @media (--md-scr) {
+    margin: 10rem auto;
+  }
+
+  @media (--sm-scr) {
+    margin: 6rem auto;
+  }
+}
+
+.heading {
+  font-size: 3rem;
+  margin: 2rem 0;
+
+  @media (--sm-scr) {
+    font-size: 2rem;
+  }
+}
+
+.copy {
+  font-size: 1.75rem;
+
+  @media (--sm-scr) {
+    font-size: 1.1rem;
+  }
+}

src/pages/subscriber-thank-you.tsx

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+import ThankYou from '../components/ThankYou'
+export default ThankYou