Unsimplify alias precedence handling

As discussed with hoellen on matrix, since postfix indeed supports including
the recipient delimiter character in a verbatim alias, we should support so too
— and handle its precedence correctly. The clearer and simpler formulation of
the precedence-clauses are credit to @hoellen. Thanks!

core/admin/mailu/models.py

@@ -260,14 +260,22 @@ class Email(object):
 
     @classmethod
     def resolve_destination(cls, localpart, domain_name, ignore_forward_keep=False):
-        localpart_maybe_split = localpart
+        localpart_stripped = None
+        stripped_alias = None
+
         if os.environ.get('RECIPIENT_DELIMITER') in localpart:
-            localpart_maybe_split = localpart.rsplit(os.environ.get('RECIPIENT_DELIMITER'), 1)[0]
+            localpart_stripped = localpart.rsplit(os.environ.get('RECIPIENT_DELIMITER'), 1)[0]
 
-        alias = Alias.resolve(localpart_maybe_split, domain_name)
+        pure_alias = Alias.resolve(localpart, domain_name)
+        pure_alias_has_wildcard = pure_alias and '%' in pure_alias.email and pure_alias.wildcard
+        stripped_alias = Alias.resolve(localpart_stripped, domain_name)
 
-        if alias:
+        if pure_alias and not pure_alias_has_wildcard:
-            return alias.destination
+            return pure_alias.destination
+        elif stripped_alias:
+            return stripped_alias.destination
+        elif pure_alias:
+            return pure_alias.destination
 
         user = User.query.get('{}@{}'.format(localpart, domain_name))
         if not user and localpart_stripped: