Improve mobile composer behaviour

- Don't scroll to the bottom of the discussion when selecting "Reply"
  from the menu if the composer is in full screen mode (ie. on mobile).
  ref #1271

- After posting a reply, scroll to the end of the discussion

- Reduce the textarea height - previously it was 100vh, but this doesn't
  account for the height of the iOS keyboard, so I've just arbitrarily
  chosen 300px instead. There may be a better solution for this.
  ref #1269

js/forum/src/utils/DiscussionControls.js

@@ -162,7 +162,7 @@ export default {
         }
         app.composer.show();
 
-        if (goToLast && app.viewingDiscussion(this)) {
+        if (goToLast && app.viewingDiscussion(this) && ! app.composer.isFullScreen()) {
           app.current.stream.goToNumber('reply');
         }