send_email: Improve configurability for outgoing email sender name.

Currently, the sender names for outgoing emails sent by Zulip
are hardcoded. It should be configurable for self-hosted systems.

This commit makes the 'Zulip' part a variable in the following
email sender names: 'Zulip Account Security', 'Zulip Digest',
and 'Zulip Notifications' by introducing a settings variable
'SERVICE_NAME' with the default value as f"{EXTERNAL_HOST} Zulip".

Fixes: #23857

zproject/default_settings.py

@@ -485,6 +485,8 @@ WELCOME_EMAIL_SENDER: Optional[Dict[str, str]] = None
 
 # Whether to send periodic digests of activity.
 SEND_DIGEST_EMAILS = True
+# The variable part of email sender names to be used for outgoing emails.
+INSTALLATION_NAME = EXTERNAL_HOST
 
 # Used to change the Zulip logo in portico pages.
 CUSTOM_LOGO_URL: Optional[str] = None

zproject/prod_settings_template.py

@@ -101,6 +101,13 @@ EXTERNAL_HOST = "zulip.example.com"
 ## confirmation emails when ADD_TOKENS_TO_NOREPLY_ADDRESS=False.
 # NOREPLY_EMAIL_ADDRESS = "noreply@example.com"
 
+## Emails sent by the Zulip server will use a sender name starting
+## with INSTALLATION_NAME. The default is EXTERNAL_HOST. If INSTALLATION_NAME is
+## "zulip.example.com", email senders names will include:
+## * "zulip.example.com notifications" (message notification emails).
+## * "zulip.example.com account security" (account security emails).
+# INSTALLATION_NAME = "My Zulip Server"
+
 ## Many countries and bulk mailers require certain types of email to display
 ## a physical mailing address to comply with anti-spam legislation.
 ## Non-commercial and non-public-facing installations are unlikely to need