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

@@ -40,6 +40,7 @@ test('Screenshot Postbox', async () => {
   await expect(await page.evaluate(
     () => localStorage.getItem("isso-author")))
     .toBe(null);
+  await page.waitForSelector('#isso-thread');
   const thread = await page.$('#isso-thread');
   await thread.screenshot({
     path: SCREENSHOTS_PATH + '/postbox.png'