Disable revalidation button on status UI if PR is closed

Mentioned in a ticket on the helpdesk, we should disable the revalidation button for the Github commit status page when the PR is in a non open state. While there is a toast notification indicating that it won't ever run, it's easy to miss and it's just easier to remove the option.

added Bug prioritylow severitylow team:webdev webdev:java labels

mentioned in issue eclipsefdn/helpdesk#4780 (closed)

Also adding a timestamp when the last test ran would be nice to make the 15 minutes grace period transparent

assigned to @malowe

added statewip label

mentioned in merge request !219 (merged)

added statereview label and removed statewip label