add login by email + feature flag

docs/config.schema.json

@@ -2504,6 +2504,10 @@
         "account_deactivation_allowed": {
           "description": "Whether users are allowed to delete their own account. Defaults to `true`.",
           "type": "boolean"
+        },
+        "login_with_email_allowed": {
+          "description": "Whether users can log in with their email address. Defaults to `false`.\n\nThis has no effect if password login is disabled.",
+          "type": "boolean"
         }
       }
     },

docs/reference/configuration.md

@@ -314,6 +314,12 @@ account:
   #
   # Defaults to `true`.
   account_deactivation_allowed: true
+
+  # Whether users can log in with their email address.
+  #
+  # Defaults to `false`.
+  # This has no effect if password login is disabled.
+  login_with_email_allowed: false
 ```
 
 ## `captcha`