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/management.py

@@ -110,7 +110,8 @@ You can use the command list_realms to find ID of the realms in this server."""
         # throw an error if they don't exist.
         if realm is not None:
             try:
-                return UserProfile.objects.select_related().get(email__iexact=email.strip(), realm=realm)
+                return UserProfile.objects.select_related().get(
+                    delivery_email__iexact=email.strip(), realm=realm)
             except UserProfile.DoesNotExist:
                 raise CommandError("The realm '%s' does not contain a user with email '%s'" % (realm, email))
 
@@ -118,7 +119,7 @@ You can use the command list_realms to find ID of the realms in this server."""
         # optimistically try to see if there is exactly one user with
         # that email; if so, we'll return it.
         try:
-            return UserProfile.objects.select_related().get(email__iexact=email.strip())
+            return UserProfile.objects.select_related().get(delivery_email__iexact=email.strip())
         except MultipleObjectsReturned:
             raise CommandError("This Zulip server contains multiple users with that email " +
                                "(in different realms); please pass `--realm` "