[fix bug 1003347] popup share button on the firefox download page cut off

Fixes the FB share button being cut off by:

* Making the scene "theater" container 20px taller than before.
* Moving the share button to just above the "thank you" message.

This bug isn't easy to reproduce locally due to the way the FB like button works, so I've put it up on demo3:

https://www-demo3.allizom.org/en-US/firefox/new/

Attached a [screenshot of the fix](https://bug1003347.bugzilla.mozilla.org/attachment.cgi?id=8434024).