Feature Request: Redirect /posts to /wp-admin/edit.php, showing a site-picker if needed

### What

When people go to https://wordpress.com/posts they just get a list of posts from across all of their sites (unless suffering from #100852)

Could we show a site-picker instead that sends folks to /wp-admin/edit.php for the correct site?

### Why

Links to https://wordpress.com/posts and https://wordpress.com/pages exist outside the admin side bar.
They're all over our forums, in peoples bookmarks, in support emails folks refer back to, and so on.

Knowing that, I'd hoped that through the unentangling project we'd redirect calypso links when possible, but this one doesn't redirect.

### How

If someone goes to https://wordpress.com/posts have them pick a site if needed, then show the wp-admin post list for that site.

It would be nice to do that with /pages, too.

Start by tracing the routing for /posts and the flow that selects a site. Compare the requested behavior with how /pages currently works, then verify that a selected site reaches its wp-admin post list and that multi-site users see a picker before the redirect.

Written by the indexing model from the issue text.