Implement box preset export and import to use them as templates

Implement an option to download a JSON with the current box layout as a custom preset, and the ability to import one and apply it to the current boxes.

This way, it will be easy to :

• download anyone's box layout and apply it to your own boxes.
• modify your layout manually and import it again
• or just keep a back up of the layout

The downloaded JSON won't include any pokemon data, only if they are shiny or not.