Merge branch 'fix/13517-impossible-subsequent-issuable-edits-in-sidebar' into 'master'

Fix a bug preventing from doing subsequent edits in any Issuable sidebar

Fixes #13517. Fixes #12572.

### Issue

![impossible-subsequent-issue-edits-in-sidebar](/uploads/bd45c0effb46848641932d9cd38d3fe2/impossible-subsequent-issue-edits-in-sidebar.gif)

---

### Merge request

![impossible-subsequent-mr-edits-in-sidebar](/uploads/b4e1e3e93df4ac0a3d962df8b6139de9/impossible-subsequent-mr-edits-in-sidebar.gif)

/cc @jschatz1 @sytses 

See merge request !2848

app/views/projects/issues/update.js.haml

 $('aside.right-sidebar')[0].outerHTML = "#{escape_javascript(render 'shared/issuable/sidebar', issuable: @issue)}";
 $('aside.right-sidebar').effect('highlight');
-new Issue();
\ No newline at end of file
+new IssuableContext();

app/views/projects/merge_requests/update.js.haml

-$('aside.right-sidebar')[0].outerHTML= "#{escape_javascript(render 'shared/issuable/sidebar', issuable: @merge_request)}";
-$('aside.right-sidebar').effect('highlight')
-merge_request = new MergeRequest();
+$('aside.right-sidebar')[0].outerHTML = "#{escape_javascript(render 'shared/issuable/sidebar', issuable: @merge_request)}";
+$('aside.right-sidebar').effect('highlight');
+new IssuableContext();