email: Convert accounts code to use delivery_email.

A key part of this is the new helper, get_user_by_delivery_email.  Its
verbose name is important for clarity; it should help avoid blind
copy-pasting of get_user (which we'll also want to rename).
Unfortunately, it requires detailed understanding of the context to
figure out which one to use; each is used in about half of call sites.

Another important note is that this PR doesn't migrate get_user calls
in the tests except where not doing so would cause the tests to fail.
This probably deserves a follow-up refactor to avoid bugs here.

zerver/lib/notifications.py

@@ -508,7 +508,7 @@ def enqueue_welcome_emails(user: UserProfile, realm_creation: bool=False) -> Non
         from_address = FromAddress.SUPPORT
 
     other_account_count = UserProfile.objects.filter(
-        email__iexact=user.email).exclude(id=user.id).count()
+        delivery_email__iexact=user.delivery_email).exclude(id=user.id).count()
     unsubscribe_link = one_click_unsubscribe_link(user, "welcome")
     context = common_context(user)
     context.update({