Display CI job backlog for each runner

Right now, there is very little information available to users as to how soon a CI build might start (especially on GitLab.com). Often times, users are left wondering why jobs won't run. Displaying information about how many builds are waiting in the queue, the age of the oldest pending build, and number of builds currently running would be very helpful.

Ideally, the backlog information would be displayed individually for each runner (checking for matching tags), but this might be too hard on resources. At the very least, this information should be displayed for the shared runners as a whole. Checks will need to be made to make sure builds aren't counted that will never be run due to tags not matching (causing inflated counts).

An example of what this might look like (for per-runner information):

/cc @ayufan

Related: status-gitlab-com#5 (closed)