Implement Migration classes for D8 upgrade

Once we have a layout plugin type (#2296431: Implement layout plugin type (or get it from somewhere)) and style plugin type (#2296437: Implement style plugin type), we can start experimenting with Migration classes to upgrade D7 page_manager pages to D8.

They'll probably take a couple mapping arrays (to convert plugin names and settings from D7 -> D8) as an argument, so we don't really have to worry about having the actual plugins ported to implement this. It'll just be about reading through the D7 tables and creating the D8 configuration entities, copying the settings array for each of the plugins into D8.

We can worry about actually creating some default mappings when we start porting all the plugins.