Grant editor perms to all current maslow users

Before the [introduction of permissions into Maslow](https://github.com/alphagov/maslow/pull/96), anyone with a `signin`
permission can perform any change within the app. Once permissions are live,
then anyone with the `signin` permission can view only.

In order to preserve editing for the users who have been entrusted with that
ability up to now, all Maslow users get the extra `editor` permission.