Mailchimp block: add option to pick your own audience group from the front-end

[jeherve]

The Mailchimp block includes options for the post author to pick an audience group for all visitors signing up using that block.

However, the Mailchimp docs suggest a different approach:

Groups are specialized audience fields that let contacts self-categorize based on interests or preferences using uniform responses you set.
-- https://mailchimp.com/help/create-new-audience-group/

The keyword there is "self-categorize", that is, the person who's signing up for the email list should themselves be able to select the group they want to be in (if the list has groups).

We could adapt our block to surface the group options in the actual block to let the visitors choose.

[stale]

This issue has been marked as stale. This happened because:

• It has been inactive in the past 6 months.
• It hasn’t been labeled `[Pri] Blocker`, `[Pri] High`.

No further action is needed. But it's worth checking if this ticket has clear reproduction steps and it is still reproducible. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.