Perform switch from Postmark to SparkPost

[begedin]

Problem

We want to change our email provider to SparkPost, since it better suits our needs.

Subtasks

• [ ] Implement testing infrastructure
• [ ] Make the switch
• [ ] Create Issue: Optimize email sending to make use of sparkpost's multiple recipient support

References

Might make #1330 obsolete or require it to be reworded