Part 2 of project milestone resource refactoring

This commit moves attributes of a project milestone resource to the
main class so that the EE one can inherit it, and focus only on EE
specific stuff.

And one of the attributes is being removed since it wasn't in use.

Also, it prefers to be explicit about how fabrication happens (via
API) than being implicit (fabricate!)

Finally, this change saves us six lines of code.

qa/qa/ee/resource/project_milestone.rb

@@ -6,15 +6,6 @@ module QA
       class ProjectMilestone < QA::Resource::ProjectMilestone
         attr_writer :start_date, :due_date
 
-        attribute :id
-        attribute :iid
-
-        attribute :project do
-          QA::Resource::Project.fabricate_via_api! do |resource|
-            resource.name = 'project-with-milestone'
-          end
-        end
-
         def api_post_body
           {
             title: title

qa/qa/resource/project_milestone.rb

@@ -3,10 +3,13 @@
 module QA
   module Resource
     class ProjectMilestone < Base
+      attribute :id
       attribute :title
 
       attribute :project do
-        Project.fabricate!
+        Project.fabricate_via_api! do |resource|
+          resource.name = 'project-with-milestone'
+        end
       end
 
       def initialize