Implement form subscriptions

[fritzmg]

Often times our customers also want to enable the possibility to subscribe to a Mailchimp newsletter in regular Contao forms. i.e. the form is collecting other data (possibly stored with terminal42/contao-leads for example) but then the email address should also be subscribed to a Mailchimp newsletter (may be when clicking a checkbox).

So far we have implemented our own processFormData listeners in these projects, but since we need it in yet another project now, I thought it would be good to have this integrated directly into this extension :)

Usage:

1. Set up the Mailchimp API credentials first as usual.
2. Enable in the form's settings.
3. Select a Mailchimp list.
4. Optionally activate interest groups. Every subscription will have these interest groups enabled.
5. Optionally set a confirm form field's name. The subscription will only be done, if the value of that form field is not empty.
6. Optionally enable Mailchimp double opt-in.
7. Define the merge tags. The "Key" is the Mailchimp merge tag while the "Value" is the form field's name. You need at least the EMAIL merge tag in order to indicate which form field holds the email address. Also the Mailchimp API requires at least the EMAIL merge tag.

Notes:

• I have put the whole functionality into a single service, i.e. this one service implements all the necessary callbacks/hooks. This way everything is self contained. But I can split it up if preferred.
• I used PHP language files since they are much easier to manage (if not using external translator tools), I hope you don't mind 😁
• oneup/mailchimp-api-v3 currently does not provide much error information when subscribing. So the logging can only log whether it was successful or not - without access to the reason when not successful.
• PR is a draft for now as we are still testing this in a live instance.

[bytehead]

oneup/mailchimp-api-v3 currently does not provide much error information when subscribing. So the logging can only log whether it was successful or not - without access to the reason when not successful.

I'm all in to update the API library to fit for future needs. 👍 Probably needs a update/rewrite anyway (PHP 8).

[fritzmg]

Took a while to properly test this with the customer, but this is RTM now 😁

[bytehead]

Thank you @fritzmg! Really nice feature!

[bytehead]

Would you mind to add some usage notes to the README.md as well?

[fritzmg]

Would you mind to add some usage notes to the README.md as well?

Ah, yes I meant to, but forgot.