Simplify ADMINS and MANAGERS email list settings

[Mike Edmunds]

Django should allow the ​ADMINS and ​MANAGERS settings to be simple lists of email addresses, like this:

# Doesn't work in Django through 5.2:
ADMINS = ["john@example.com", "mary@example.com"]

Right now, those settings both expect a list of tuples of (full name, email address). But the name part is ignored in both ​mail_admins() and ​mail_managers()—and it's ​been ignored for at least 20 years (as far back as the Git history goes):

ADMINS = [("name is ignored", "john@example.com"), ("ignored", "mary@example.com)]

#30604 added an error on detecting the (reasonable) mistake of trying to set ADMINS or MANAGERS to a simple list of addresses as in first example.

This ticket proposes instead allowing—and documenting—simple address lists in these settings. For compatibility, Django would also continue to support the list-of-tuples form (with the name field ignored).

Incidentally, if you do want to include a display-name in the admins/managers recipients lists, that can be done with the RFC 5322 "name" <addr> format (which is accepted by all the django.core.mail APIs):

# Works in Django 5.2 and earlier:
ADMINS = [
    ("ignored", "John <john@example.com>"), 
    ("ignored", '"Mary, IT (Ops)" <mary@example.com>'),
]

# Also allowed after this proposed change:
ADMINS = [
    "John <john@example.com>", 
    '"Mary, IT (Ops)" <mary@example.com>',
]