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/users.py

@@ -185,7 +185,7 @@ def access_user_by_id(user_profile: UserProfile, user_id: int,
 def get_accounts_for_email(email: str) -> List[Dict[str, Optional[str]]]:
     if settings.PRODUCTION:  # nocoverage
         return []
-    profiles = UserProfile.objects.select_related('realm').filter(email__iexact=email.strip(),
+    profiles = UserProfile.objects.select_related('realm').filter(delivery_email__iexact=email.strip(),
                                                                   is_active=True,
                                                                   is_bot=False,
                                                                   realm__deactivated=False)