Review security issues and plot vulnerable-since times

We're working on more and more security issues. This is good, in the main: it means that GitLab is a more secure product. The concern is about the workload, and there are two broad models that could explain the volume of security issues:

1. We keep introducing new security issues that require fixing.
2. We have a historical, mostly-invisible backlog of security issues that we're working through right now. (Invisible because we don't know about them until they are reported.)

The truth is probably somewhere between the two, of course, but it would be useful to know how far it is in either direction.

To find out the answer, I should be able to look at the security issues we've addressed, look at when the issue was introduced, and plot that out somewhere.