Helping project maintainers manage RTBC issues on a first-in/first-out basis

We will be adding a last issue status change timestamp to issue listings so that project maintainers can sort RTBC issues by when they were put into that status. This will allow them to address RTBC issues on a first-in/first-out basis, rather than bumping the issues that simply have more recent comments.

Production outage: No
Change affects: Drupal.org issue queues
Deployment schedule: week of 2017-09-11
Backout plan: revert or fix in place