add confirm prompt for template upload via URL

app/javascript/elements/confirm_upload.js

@@ -0,0 +1,27 @@
+import { target, targetable } from '@github/catalyst/lib/targetable'
+
+export default targetable(class extends HTMLElement {
+  static [target.static] = [
+    'prompt',
+    'processing',
+    'logo'
+  ]
+
+  connectedCallback () {
+    this.form.addEventListener('submit', this.onSubmit)
+  }
+
+  disconnectedCallback () {
+    this.form.removeEventListener('submit', this.onSubmit)
+  }
+
+  onSubmit = () => {
+    this.prompt.classList.add('hidden')
+    this.processing.classList.remove('hidden')
+    this.logo.classList.add('animate-bounce')
+  }
+
+  get form () {
+    return this.querySelector('form')
+  }
+})