Unify newsletters + survey

[gedankenstuecke]

After a small mishap today: Right now we have the survey & the newsletter feature. Which can lead to accidentally sending the wrong one if one isn't careful. Instead we should probably

• go back to having a single system for both
• use a reply-able address for the newsletter as well (right now the newsletter uses donotreply@opensnp.org, while the survey uses survey@opensnp.org).

This directly relates to #405.