I've restarted Jenkins, which may have lost track of any builds that
were in progress at the time - sorry! Please re-test any PRs affected.
The JVM heap usage was getting quite critical and as it's sized a little
over the amount of physical RAM on the VM, the GC was causing heavy
swapping, causing the UI to become unresponsive. I've added a regular
scheduled safe restart now at around 3am UTC to help keep it in check
and will look at moving it to a bigger VM soon with more heap if that
doesn't help.