💻 Allow existing users to subscribe to the newsletter

[boryanagoncharenko]

Context
Currently, a user can subscribe to our newsletter only when they create their account. This has 2 consequences:

• the code that subscribes the users assumes they are brand new accounts with no classes, customizations or history and this includes the changes in PR 📜 Add tags to mailchimp users #6080, so the code does not check if it should add any relevant tags.
• the existing audience in Mailchimp does not have tags that indicate if the user has created classes, customizations etc.

Scope
Create a subscribe endpoint that users can use after they have created their account. This means that this endpoints should check whether the user has done all actions of interest and should update their Mailchimp tags accordingly. Also, this means we might need to revise the triggers of the journeys.

Add an elegant (or at least not annoying) way to offer the users to subscribe. Probably we want to offer this only to logged in teachers and in a non-intrusive way. We could offer this option on the Hedy landing page and in a separate section in the user profile page.

[AnneliesVlaar]

In regards to Issue #3995 (Mailchimp issue), we want to have a button somewhere on the teacher page so that teachers can subscribe to the newsletter. Right now, teachers can only opt in on the newsletter when creating their account, there is no way to join later. @AnneliesVlaar or @rix0rrr can you think of a nice place to add this button?

Sure I think it can go as a checkbox in My account under my personal settings.

[AnneliesVlaar]

Information from @boryanagoncharenko: In short, we cannot offer a checkbox in the profile because we cannot populate its value correctly. When a person subscribes to us, we tell Mailchimp that the user opted in and Mailchimp sets their status to "Subscribed". The way for them to unsubscribe is to click the "Unsubscribe" link in any email they receive from us. Alternatively they should be able to use a Mailchimp-hosted sign up form that we created (but unfortunately I do not have access to it or even verify it exists). When they unsubscribe, we do not get a notification of anything - Mailchimps silently sets their status to "Unsubscribed". Technically, subscribing a user is a one-way street, so probably it should be a button rather than a checkbox.

Here is the tricky part. If a user has unsubscribed, it seems that WE cannot resubscribe them via code. We can only send them a link to this Mailchimp-hosted signup form that we created and they need to resubscribe themselves. This is my source: https://mailchimp.com/help/resubscribe-a-contact/