Do not reload page after adding comments in Pull Request reviews (#13877)

Fixed #8861 
* use ajax on PR review page

* handle review comments

* extract duplicate code

FetchCodeCommentsByLine was initially more or less copied from fetchCodeCommentsByReview. Now they both use a common findCodeComments function instead

* use the Engine that was passed into the method

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>

routers/routes/macaron.go

@@ -856,6 +856,7 @@ func RegisterMacaronRoutes(m *macaron.Macaron) {
 			m.Group("/files", func() {
 				m.Get("", context.RepoRef(), repo.SetEditorconfigIfExists, repo.SetDiffViewStyle, repo.SetWhitespaceBehavior, repo.ViewPullFiles)
 				m.Group("/reviews", func() {
+					m.Get("/new_comment", repo.RenderNewCodeCommentForm)
 					m.Post("/comments", bindIgnErr(auth.CodeCommentForm{}), repo.CreateCodeComment)
 					m.Post("/submit", bindIgnErr(auth.SubmitReviewForm{}), repo.SubmitReview)
 				}, context.RepoMustNotBeArchived())