* this is quite a bit simpler than cobbling together the message in a
  normal python function

* previously, i forgot to override that configuration item after
  calculating its value
* if not set as configuration explicitly, Invenio-RDM-Records would then
  set a default value of "127.0.0.1:5000" for it

* this brings it more in line with the request notifications
* previously we were restricted to emails, now it will be whatever
  notification backends we use
* also remove the mail templates - they now live in code until we
  enhance the notification backend to be more flexible w.r.t. the
  rendered templates

* 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

* also start kicking out infrastructure for the "trusted-publisher" role
  which hasn't been used in a good while now
* instead, if curation isn't enabled, it's up to the admins to publish
  records