refactor: Newsletter
refactor: Newsletter

* Re-send Newsletter only to the recipients who haven't received it yet (Check Email Queue before setting email recipients)
* Re-structed code, broken larger methods to "byte" sized "bits" (hehe)
* Added validations for recipients list
* Added Newsletter.newsletter_recipients property
* Used newer APIs and Fixed namespaces for usages/imports
* Added convenience methods for Newsletter APIs
* Added type annotations for all(?) methods

almeidapaulopt/frappe,yashodhank/frappe,mhbu50/frappe,almeidapaulopt/frappe,yashodhank/frappe,StrellaGroup/frappe,frappe/frappe,mhbu50/frappe,StrellaGroup/frappe,mhbu50/frappe,frappe/frappe,yashodhank/frappe,StrellaGroup/frappe,yashodhank/frappe,almeidapaulopt/frappe,frappe/frappe,mhbu50/frappe,almeidapaulopt/frappe