WIP Limits v2

Now that we've ditched the distinction between project and maintenance work, we've been forced to tackle the WIP limit again.

Up until now we'd set a WIP limit on the project work, but not on maintenance.

Now that the work is broken into smaller chunks, it's actually become easier to set a single limit based on uniform speed.

Finally, limits!

Rather than try to figure out what the limit should be, I've just slapped on a limit that reflects the volume we've been handling up until now.

Based on what I'd read in "Kanban and Scrum - making the most of both", I'm expecting the limit to change as we gain more experience - so I didn't want to spend a huge amount of time now.