Don't wait on persona if we are using Firefox Accounts fallback (bug 1039868)

https://bugzilla.mozilla.org/show_bug.cgi?id=1039868

We'll also need a zamboni PR to not include the persona shim in the template if the waffle for firefox accounts is on, but that's [a different bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1038985).