User shouldn't be able to approve or reject his/her own PR (#4729)

* Make sure author cannot reject/approve their own PR

* Disable buttons in templates too

* Remove unneccessary if check since the switch below catches it

* Fix IsOwner check

* Update template and remove new template variable

* Add alert template and redirect to diff page on review failure

* Redirect to files diff as a little update to #4632

templates/repo/pulls/files.tmpl

@@ -11,6 +11,7 @@
 		<div class="ui divider"></div>
 		{{template "repo/issue/view_title" .}}
 		{{template "repo/pulls/tab_menu" .}}
+		{{template "base/alert" .}}
 		<div class="ui bottom attached tab pull segment active">
 			{{template "repo/diff/box" .}}
 		</div>