[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/milestone.yml

@@ -21,3 +21,11 @@
   content: content3
   is_closed: true
   num_issues: 0
+
+-
+  id: 4
+  repo_id: 42
+  name: milestone of repo42
+  content: content random
+  is_closed: false
+  num_issues: 0