Add a "Last Rendered" view to the webapp.
The Manager now stores (in the database) which job was the last
recipient of a rendered image, and serves that to the appropriate
OpenAPI endpoint.
A new SocketIO subscription + accompanying room makes it possible for
the web interface to receive all rendered images (if they survive the
queue, which discards images when it gets too full).
It doesn't update automatically yet (still needs SocketIO notifications),
and there is no check yet for whether there is actually any last-rendered
image at all, but at least there is a component that shows the image.
When data is updated, resize columns in the job/task/worker tables. For
example, status change requests of Workers require more space, for example
going from `awake` to `awake → offline`.
Having only the status dot was hard to read. It requires you to learn &
remember the different colours, or to mouse-over and wait to see the
tooltip. For accessibility, we shouldn't be using just the colour to
convey information in the interface.
The tasks table resize function is called via `this.$nextTick()`, which
means that the component can actually already be unmounted by the time
the actual function call is performed. This is now detected & handled.
