Bug 910014 - Success message on Firefox & You signup

When you signup for the Firefox & You newsletter at
https://www.mozilla.org/firefox/os/ or
https://www.mozilla.org/firefox/fx, change the success
message.
Merge pull request #1200 from dpoirier/bug-910014-ff-and-you-signup-message

Bug 910014 - Success message on Firefox & You signup