[API] ListIssues add filter for milestones (#10148)

* Refactor Issue Filter Func

* ListIssues add filter for milestones

* as per @lafriks

* documentation ...

models/fixtures/issue.yml

@@ -32,6 +32,7 @@
   poster_id: 1
   name: issue3
   content: content for the third issue
+  milestone_id: 3
   is_closed: false
   is_pull: true
   created_unix: 946684820

models/fixtures/milestone.yml

@@ -20,7 +20,7 @@
   name: milestone3
   content: content3
   is_closed: true
-  num_issues: 0
+  num_issues: 1
 
 -
   id: 4