Site Migration: New step to choose between importing and migrating

[markbiek]

The design of this step is TBD so we'll start with a placeholder UI for now.

While the design of this step is still in progress, we've been imagining it as some kind of two-column layout which illustrates the differences between "Importing" and "Migrating".

If the user clicks "Import", they'll be take to the existing import flow.

For the Migration side, we'll want to check the user's plan. If it's a plugin-capable plan, we'll show a "Migrate my site" button which takes them into the /setup/site-migration flow.

If they don't have a plugin-capable plan yet, we'll show an "Upgrade to Migrate" button which takes them to the upgrade screen we're building in #87605.

Once they've upgraded, they'll continue into the /setup/site-migration flow.

Context: paYKcK-45p-p2#comment-3025

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• [ ] paYKcK-45p-p2#comment-3025

[markbiek]

The remaining work on this step is happening in #87905.