closes https://github.com/TryGhost/Team/issues/506
- launch wizard and brand settings both shared the same design and behaviour for customising brand settings but used duplicated code that had diverged
- extracted the more up-to-date behaviour from the launch wizard into a component
- updated brand settings and launch wizard to use the new component
changes to brand settings modal behaviour:
- preview is no longer interactive
- switches to using iframe contents replacement instead of `postMessage`
