Use issue number/index instead of id for API URL. (#3298)

Using the API calls, the issue objects retrieved by
/repos/{owner}/{repo}/issues
contains the wrong value in the "url" field:
it uses the "id" value instead of the "number" value.

For instance, in a new repo in try.gitea.io with just one issue:
[{
  "id": 896,
  "url": "https://try.gitea.io/api/v1/repos/AlbertoGP/uno/issues/896",
  "number": 1,
   ...
}]

The given URL does not work, but if you replace the "896" by "1" it
does.

models/issue.go

@@ -248,7 +248,7 @@ func (issue *Issue) GetIsRead(userID int64) error {
 
 // APIURL returns the absolute APIURL to this issue.
 func (issue *Issue) APIURL() string {
-	return issue.Repo.APIURL() + "/" + path.Join("issues", fmt.Sprint(issue.ID))
+	return issue.Repo.APIURL() + "/" + path.Join("issues", fmt.Sprint(issue.Index))
 }
 
 // HTMLURL returns the absolute URL to this issue.