Mark PR reviews as stale at push and allow to dismiss stale approvals (#9532)

Fix #5997.

If a push causes the patch/diff of a PR towards target branch to change, all existing reviews for the PR will be set and shown as stale.
New branch protection option to dismiss stale approvals are added.
To show that a review is not based on the latest PR changes, an hourglass is shown

templates/repo/diff/comment_form.tmpl

@@ -4,6 +4,7 @@
 	{{end}}
 	<form class="ui form {{if $.hidden}}hide comment-form comment-form-reply{{end}}" action="{{$.root.Issue.HTMLURL}}/files/reviews/comments" method="post">
 	{{$.root.CsrfTokenHtml}}
+		<input type="hidden" name="latest_commit_id" value="{{$.root.AfterCommitID}}"/>
 		<input type="hidden" name="side" value="{{if $.Side}}{{$.Side}}{{end}}">
 		<input type="hidden" name="line" value="{{if $.Line}}{{$.Line}}{{end}}">
 		<input type="hidden" name="path" value="{{if $.File}}{{$.File}}{{end}}">

templates/repo/diff/new_review.tmpl

@@ -7,6 +7,7 @@
 		<div class="ui clearing segment">
 			<form class="ui form" action="{{.Link}}/reviews/submit" method="post">
 			{{.CsrfTokenHtml}}
+			    <input type="hidden" name="commit_id" value="{{.AfterCommitID}}"/>
 				<i class="ui right floated link icon close"></i>
 				<div class="header">
 				{{$.i18n.Tr "repo.diff.review.header"}}

templates/repo/issue/view_content/pull.tmpl

@@ -13,6 +13,11 @@
 							{{else}}grey{{end}}">
 							<span class="octicon octicon-{{.Type.Icon}}"></span>
 						</span>
+						{{if .Stale}}
+						<span class="type-icon text grey">
+							<i class="octicon icon fa-hourglass-end"></i>
+						</span>
+						{{end}}
 						<a class="ui avatar image" href="{{.Reviewer.HomeLink}}">
 							<img src="{{.Reviewer.RelAvatarLink}}">
 						</a>

templates/repo/settings/protected_branch.tmpl

@@ -211,6 +211,14 @@
 							<p class="help">{{.i18n.Tr "repo.settings.block_rejected_reviews_desc"}}</p>
 						</div>
 					</div>					
+					<div class="field">
+						<div class="ui checkbox">
+							<input name="dismiss_stale_approvals" type="checkbox" {{if .Branch.DismissStaleApprovals}}checked{{end}}>
+							<label for="dismiss_stale_approvals">{{.i18n.Tr "repo.settings.dismiss_stale_approvals"}}</label>
+							<p class="help">{{.i18n.Tr "repo.settings.dismiss_stale_approvals_desc"}}</p>
+						</div>
+					</div>
+
 				</div>
 
 				<div class="ui divider"></div>