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 #913

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

@@ -73,11 +73,13 @@ test('Screenshot with inserted comment', async () => {
     expect(page).toClick('.isso-post-action > input[type=submit]'),
   ]);
 
+  await page.waitForSelector('#isso-1');
   const rendered_comment = await page.$('#isso-1');
   await rendered_comment.screenshot({
     path: SCREENSHOTS_PATH + '/comment.png'
   });
 
+  await page.waitForSelector('#isso-thread');
   const thread = await page.$('#isso-thread');
   await thread.screenshot({
     path: SCREENSHOTS_PATH + '/thread.png'