[Architecture] Figure out which components need migration

In the [PoC branch](https://github.com/Automattic/woocommerce-services/compare/tweak/poc-architecture-meetup-v2) we still have `wp-calypso` as a folder (but not a submodule). It isn't really calypso, just a very small subset of it. Refer to `WooCommerce Services Architecture` p2 for more info, e.g. list of components.

We have about 20~ non-Gutenberg components in `client/extensions/woocommerce/woocommerce-services/components`. We need to:

1. Figure out which of those already have a good replacement in `@wordpress/components` - in which case we probably want to create issues for migrating the code (e.g. whatever we need to change from using Calypso's `Button` to using Gutenberg's `Button`)
2. For those that don't have a replacement in `@wordpress/components` we have two options:
- Make this extension contain the component itself
- Contribute back to Gutenberg

We can discuss it at the roundtable.

Review the PoC branch and the `WooCommerce Services Architecture` document, page 2, then inventory the components in `client/extensions/woocommerce/woocommerce-services/components`. Compare each non-Gutenberg component with `@wordpress/components`. Done means identifying replacements and recording whether each unmatched component should stay in the extension or be contributed to Gutenberg, with follow-up issues as needed.

Written by the indexing model from the issue text.