Provide more obvious way to close discussion form

[jaeddy]

Currently, after a user clicks + START A NEW DISCUSSION button, the only way to hide the form is to click the same button again.

This isn't super intuitive. Might be nice to have a Cancel button or just an X of some sort.

[tschaffter]

@jaeddy 39429e4 adds a button to discard the new thread form.