💻 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.