Mailchimp Block: Connect block from the editor

[creativecoder]

Inserting the Mailchimp block currently requires opening a new window and connecting to Mailchimp in Calypso.

Requirements

1. Users can connect the Mailchimp block without leaving the editor
2. The connection flow for Mailchimp opens in a smaller popup window, in the same way that the Instagram block works
3. The block in the editor displays a loading state while the connection window is open
4. When the connection flow is completed successfully, the block in the editor is automatically updated to be in a connected state
5. When the connection flow errors, the a notice appears at the top of the block placeholder with an error message and recommend next step
6. When the connection flow is abandoned (because the popup window is closed) the block returns to it's default placeholder state.

[creativecoder]

The Latest Instagram Posts block has an inline oauth connection flow that happens in a popup window without leaving the editor--we should be able to follow this pattern for the Mailchimp block (and any other blocks that require an oauth connection to work)