Indicate WP subject as non-writable when generated

Automatic subject generation should prevent users from
editing the subject directly. Instead it can only be updated
automatically, when other fields change in that case.

lib/api/v3/work_packages/schema/work_package_schema_representer.rb

@@ -116,7 +116,8 @@ def initialize(schema, self_link:, **context)
           schema :subject,
                  type: "String",
                  min_length: 1,
-                 max_length: 255
+                 max_length: 255,
+                 writable: ->(*) { default_writable_property(:subject) && !represented.type&.replacement_pattern_defined_for?(:subject) }
 
           schema :description,
                  type: "Formattable",