Replace Meetup.com by Mailchimp signup embedded form

@parazito and me was discussing about Meetup.com which is billing us since February, 2022. As we're not scheduling any meetup anymore, we don't see benefits by keeping paying Meetup.com subscription.

So, as Meetup.com does not provide the Members e-mail addresses we've decided to introduce a form to be able to receive subscriptions. So, when we schedule a next meetup will be possible to notify members through a free event page (Sympla/EventBrite/SplashThat).