[Backport] [API] Allow only specific Colums to be updated on Issue (#9539) (#9580)

* Fix #9189 - API Allow only specific Colums to be updated on Issue (#9539)

* dont insert "-1" in any case to issue.poster_id

* Make sure API cant override importand fields

* code format

* add Test for IssueEdit

* load milestone and return it on IssueEdit via API

* extend Test for TestAPIEditIssue

* fix TEST

* make sure Poster is loaded

* keep code format on backport as it is

models/fixtures/issue.yml

@@ -96,4 +96,17 @@
   is_closed: false
   is_pull: true
   created_unix: 946684820
-  updated_unix: 978307180
+  updated_unix: 978307180
+
+-
+  id: 9
+  repo_id: 42
+  index: 1
+  poster_id: 500
+  name: issue from deleted account
+  content: content from deleted account
+  is_closed: false
+  is_pull: false
+  created_unix: 946684830
+  updated_unix: 999307200
+  deadline_unix: 1019307200