models: Create delivery_email field in userprofile.

This commit creates a new field called delivery_email. For now, it is
exactly the same as email upon user profile creation and should stay
that way even when email is changed, and is used only for sending
outgoing email from Zulip.

The purpose of this field is to support an upcoming option where the
existing `email` field in Zulip becomes effectively the user's
"display email" address, as part of making it possible for users
actual email addresses (that can receive email, stored in the
delivery_email field) to not be available to other non-administrator
users in the organization.

Because the `email` field is used in numerous places in display code,
in the API, and in database queries, the shortest path to implementing
this "private email" feature is to keep "email" as-is in those parts
of the codebase, and just set the existing "email" ("display email")
model field to be something generated like
"username@zulip.example.com" for display purposes.

Eventually, we'll want to do further refactoring, either in the form
of having both `display_email` and `delivery_email` as fields, or
renaming "email" to "username".

zerver/lib/send_email.py

@@ -43,7 +43,7 @@ def build_email(template_prefix: str, to_user_id: Optional[int]=None,
         to_user = get_user_profile_by_id(to_user_id)
         # Change to formataddr((to_user.full_name, to_user.email)) once
         # https://github.com/zulip/zulip/issues/4676 is resolved
-        to_email = to_user.email
+        to_email = to_user.delivery_email
 
     if context is None:
         context = {}