Removed trailing whitespaces with

```sh
for file in `grep -r " $" * | cut -d : -f 1 | sort | uniq | grep -v " "`; do sed -i 's/[ \t]*$//' $file; done
```

and some handwork.

using `event.preventDefault` instead of `return false` for being more
explicit

If .btn-default isn’t removed, .btn-default and .btn-warning styles
conflict

Click handler was also attaching to “Select Attachments” button, so
modified selector to grab #issue-reply-btn by ID and submit button