Prevent both [+] and [-] icons from displaying when the snapshot name is lengthy.  Fixes EUCA-7544

Since the width between the [+] and [-] icons was fixed in the existing CSS sprite image, a new image had to be created to make this work.  Left existing sprite image as-is to avoid breaking existing background-position properties for other icons.

Fixes https://eucalyptus.atlassian.net/browse/EUCA-7544