Mailchimp Connection: Allow separate connections for separate sites

#### Is your feature request related to a problem? Please describe.
Currently, you can only have one Mailchimp account connected, per WordPress.com account.

If a user has two separate sites, they can connect them to separate lists, but only within the same Mailchimp account.

If a user wishes to have two separate sites, with two separate Mailchimp accounts, it is currently not possible to have them both connected at the same time (disconnecting one site from Mailchimp also disconnects all other sites in the same WordPress.com account)

#### Describe the solution you'd like
It would be nice to connect Mailchimp to a specific site, instead of to the WordPress.com account.

Another solution that could work is allowing multiple Mailchimp connections, per WordPress.com account, and then allow individual sites to choose which account + list to use.

#### Describe alternatives you've considered
I am going to suggest the user attempt to create a second WordPress.com account and use that to connect to their second Mailchimp account, but it seems like a lot of extra layers to manage for a fairly straightforward need.

#### Additional context
This idea came from a conversation on WordPress.com live chat 336254-hc

No files, tests, or entry points are named in the issue. Start by locating the Mailchimp connection handling and tracing how connections are associated with WordPress.com accounts and sites. Done means separate sites can use separate Mailchimp accounts without disconnecting one another, with the relevant behavior covered by tests.

Written by the indexing model from the issue text.