* by temporarily overriding the `notification.type`

* this enables users to set an alternative email address to which
  notifications will be sent, in case they don't like to use their
  institutional email address

* create dedicated modules for curations & notifications to separate the
  general notifications infrastructure from the curations-related parts
  (and requests)
* split large modules into smaller logical groups
* split tasks.py into three separate parts: curations, TISS, and general