Add support for marking/unmarking known issues via UI

[chaws]

Right now, KV team has to keep up a bunch of yaml files containing known issues they find in their day-to-day work: https://github.com/Linaro/qa-reports-known-issues

This list is getting cumbersome. While it's easy to copy the entire list from one project to another, there's not a good way to hand-mark known issues.