Next 13 - app dir and React Server Components

[DavieReid]

This is likely to be a massive refactor.

We would want to do this to take advantage of the speed improvements associated with React Server Components. The app dir is also the general direction next js is moving and we should look to align. See the roadmap.

Off the top of my head the areas this is likely to touch are:

1. layouts
2. loading indicators
3. site components
4. site middleware
5. site store

[mark-tate]

working on this branch
https://github.com/jpmorganchase/mosaic/tree/migrate-to-app-router

Completed

1. All pages load and render with layouts and components
2. all tests pass but issues running locally with rxjs fetch (which exists in current code branch)

Yet to Go.

1. Review un-used legacy middleware to see what remains, that needs moving to app-router
2. Snapshot and S3 loading
3. Review startup times, which like current version are slow on first load
4. Pull in proposed

    changes and also IBE changes