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 <[email protected]>
Co-authored-by: Lunny Xiao <[email protected]>
Co-authored-by: Lauris BH <[email protected]>