Fix comment permissions (#28213) (#28216)

backport #28213

This PR will fix some missed checks for private repositories' data on
web routes and API routes.

models/fixtures/comment.yml

@@ -66,3 +66,12 @@
   tree_path: "README.md"
   created_unix: 946684812
   invalidated: true
+
+-
+  id: 8
+  type: 0 # comment
+  poster_id: 2
+  issue_id: 4 # in repo_id 2
+  content: "comment in private pository"
+  created_unix: 946684811
+  updated_unix: 946684811

models/fixtures/issue.yml

@@ -61,7 +61,7 @@
   priority: 0
   is_closed: true
   is_pull: false
-  num_comments: 0
+  num_comments: 1
   created_unix: 946684830
   updated_unix: 978307200
   is_locked: false