Fix: Automatic notifications to consider notification sender address

[vasantvohra]

closes #4992

• Let me know if the commit message needs to be changed.
• If there's a need to add changes.rst
• As part of this minor PR,
• I upgrade my local to v3 🚀

[ThiefMaster]

Maybe removing this logic from build_default_email_template (abstract email notifications) would be a good idea as well?

[vasantvohra]

I see reply-to address there, so should I replace

event.contact_emails[0] if event.contact_emails else ''

with

config.NO_REPLY_EMAIL

?

[ThiefMaster]

no, just set it to '' all the time; that should result in indico not setting a reply-to address at all (noreply is already used in From)

[ThiefMaster]

A changelog entry would be useful for this (bugfixes section i guess, that's the most suitable one IMO):

- No longer silently fall back to the first event contact email address when sending
  registration emails where no explicit sender address has been configured (:issue:`4992`,
  :pr:`4996`, thanks :user:`vasantvohra`)

for the abstract notification templates I don't think it's needed since it only affects newly created ones