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.

zilencer/management/commands/sync_api_key.py

@@ -4,7 +4,7 @@ from typing import Any
 
 from django.core.management.base import BaseCommand
 
-from zerver.models import UserProfile, get_realm, get_user
+from zerver.models import UserProfile, get_realm, get_user_by_delivery_email
 
 class Command(BaseCommand):
     help = """Sync your API key from ~/.zuliprc into your development instance"""
@@ -21,7 +21,7 @@ class Command(BaseCommand):
 
         try:
             realm = get_realm("zulip")
-            user_profile = get_user(email, realm)
+            user_profile = get_user_by_delivery_email(email, realm)
             user_profile.api_key = api_key
             user_profile.save(update_fields=["api_key"])
         except UserProfile.DoesNotExist: