Improve code

Problem/Motivation

The webform handler I wrote is functional, but need a lot of improvements.
At the moment we can add the API-key and server prefix in the handler config.
The lists will only be available after saving and editing the handler again. The same for the field-mapping. They will only be visible after selecting and saving the correct Mailchimp List.
So, we need to implement some Ajax-callbacks to update the form.

And we can also improve the code a lot, I wrote some duplicate parts.