added locale to account

New accounts will get locale set on create, old accounts will get locale set on certificate sign. It will only be set once in this release (no updates if it changes).

Requires https://github.com/mozilla/fxa-auth-db-server/pull/53

Closes #743
Just wondering. Is this function just splitting on the comma and reconstructing it so long as the string is less than 255 chars?

And the resultant string should be 0-254 chars long (the column is VARCHAR(255))?