Add dismiss review feature (#12674)

* Add dismiss review feature

refs:
    https://github.blog/2016-10-12-dismissing-reviews-on-pull-requests/
    https://developer.github.com/v3/pulls/reviews/#dismiss-a-review-for-a-pull-request

* change modal ui and error message

* Add unDismissReview api

Signed-off-by: a1012112796 <1012112796@qq.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

routers/api/v1/api.go

@@ -891,6 +891,8 @@ func Routes() *web.Route {
 									Post(reqToken(), bind(api.SubmitPullReviewOptions{}), repo.SubmitPullReview)
 								m.Combo("/comments").
 									Get(repo.GetPullReviewComments)
+								m.Post("/dismissals", reqToken(), bind(api.DismissPullReviewOptions{}), repo.DismissPullReview)
+								m.Post("/undismissals", reqToken(), repo.UnDismissPullReview)
 							})
 						})
 						m.Combo("/requested_reviewers").