Add API to get a single issue

cf. https://developer.github.com/v3/issues/#get-a-single-issue

src/main/scala/gitbucket/core/controller/ApiController.scala

@@ -277,6 +277,19 @@ trait ApiControllerBase extends ControllerBase {
     org.scalatra.NotFound(ApiError("Rate limiting is not enabled."))
   }
 
+  /**
+   * https://developer.github.com/v3/issues/#get-a-single-issue
+   */
+  get("/api/v3/repos/:owner/:repository/issues/:id")(referrersOnly { repository =>
+    (for{
+      issueId  <- params("id").toIntOpt
+      issue <- getIssue(repository.owner, repository.name, issueId.toString)
+      openedUser <- getAccountByUserName(issue.openedUserName)
+    } yield {
+      JsonFormat(ApiIssue(issue, RepositoryName(repository), ApiUser(openedUser)))
+    }) getOrElse NotFound()
+  })
+
   /**
    * https://developer.github.com/v3/issues/comments/#list-comments-on-an-issue
    */