Use the current method of registering page type

[tf]

Makes sense. This gem only depends on pageflow 0.7 while the new api was introduced in 0.9. So technically the installation instructions would be broken for old versions. But I guess we can ignore that.

But let's also change it to use the Pageflow::BeforeAfter.page_type factory method. That pattern proved helpful to decouple from the concrete class constant.

[tilsammans]

@tf done!

[tf]

:+1: