Alter the way the MailChimp Plugin is enabled & the key is retrieved

[tudoramariei]

While this issue is two issues in one, we can't have one without the other, so they are both placed here.

• use a feature flag instead of the MAILCHIMP_KEY
• use the app interface to set the key

Use a feature flag for enabling the MailChimp plugin instead of using the actual key

The MailChimp plugin is enabled by checking if the MAILCHIMP_KEY variable has been passed.

paul/api/paul_api/paul_api/settings.py

Lines 98 to 99 in 9e6ab58

	MAILCHIMP_KEY = env("MAILCHIMP_KEY")
	PLUGIN_MAILCHIMP_ENABLED = True if MAILCHIMP_KEY else False

Instead of the key, a feature flag should be used. This ensures that the key is not stored on the server and is not passed to the server owners.

Set and store the key in the application's interface

There already exists a field in the interface to store the MailChimp key.
That field should be the only point of adding the key and allow the responsible NGO personnel to change it.