allow subscription of plugin authors to newsletter

So this PR is to get some feedback. What this does is automatically add every plugin or template author to a list at our mailchimp account (our Newsletter service). The idea is to have an easy way to contact plugin authors and inform them about new stuff (like the new extension manager) or send them general tips (eg. explain how our own HTTPClient should be preferred over other solutions).

Authors can easily unsubscribe at Mailchimp (link is in every mail they receive) and will not be automatically resubscribed.

I believe it would be beneficial for plugin authors and the DokuWiki community (because plugins will hopefully get better). But auto-subscribing people is also seen as inherently evil. So I'd like to get some more opinions on this before applying.

Please note: letting authors subscribe themselves is NOT an alternative. Its either this or no extension author newsletter.

paging @selfthinker @chris--s @klap-in @michitux