Add a column header to admin/jobs page

This column was blank before. It contain one or two pieces of
information:

* Job duration
* Job completion timestamp

For brevity, use `Timing` as the header.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62347

app/views/projects/jobs/_table.html.haml

@@ -16,7 +16,7 @@
             %th Runner
           %th Stage
           %th Name
-          %th
+          %th Timing
           %th Coverage
           %th
 

changelogs/unreleased/sh-add-header-to-jobs-admin-page.yml

@@ -0,0 +1,5 @@
+---
+title: Add a column header to admin/jobs page
+merge_request: 28837
+author:
+type: other