js: isso.js: Disable Postbox submit button on click, enable after res…

…ponse

Disable the submit button in Postbox to prevent double posting upon click. Enable the button after receiving a response from the API endpoint.

Fixes isso-comments#913

isso/js/tests/screenshots/screenshots.test.js

@@ -83,6 +83,7 @@ test('Screenshot with inserted comment', async () => {
   });
 
   // Delete comment again
+  await page.waitForSelector('#isso-1 > .isso-text-wrapper > .isso-comment-footer > .isso-delete');
   await expect(page).toClick('#isso-1 > .isso-text-wrapper > .isso-comment-footer > .isso-delete');
   // Need to click once to surface "confirm" and then again to confirm
   await expect(page).toClick('#isso-1 > .isso-text-wrapper > .isso-comment-footer > .isso-delete');