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

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.

https://github.com/code4romania/paul/blob/9e6ab58b462583f31ec7d9c0d075fda44561c5ae/api/paul_api/paul_api/settings.py#L98-L99

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.