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>

modules/auth/repo_form.go

@@ -548,6 +548,7 @@ func (f *MergePullRequestForm) Validate(ctx *macaron.Context, errs binding.Error
 
 // CodeCommentForm form for adding code comments for PRs
 type CodeCommentForm struct {
+	Origin         string `binding:"Required;In(timeline,diff)"`
 	Content        string `binding:"Required"`
 	Side           string `binding:"Required;In(previous,proposed)"`
 	Line           int64