add csp

app/javascript/elements/custom_validation.js

@@ -0,0 +1,14 @@
+export default class extends HTMLElement {
+  connectedCallback () {
+    const input = this.querySelector('input')
+    const invalidMessage = this.dataset.invalidMessage || ''
+
+    input.addEventListener('invalid', () => {
+      input.setCustomValidity(input.value ? invalidMessage : '')
+    })
+
+    input.addEventListener('input', () => {
+      input.setCustomValidity('')
+    })
+  }
+}