Better handling of KPIs with monthly data

I've freestyled the design a bit here because it seems obvious to me, but I'll double check with @henryhadlow or @mikieet that it makes sense.

In short, for monthly KPI data:

- Previously: "1 Feb 2014 to 28 Feb 2014"
- Now: "February 2014"

Also note that this doesn't actually fix the issue on the page. This is because waste carriers config is incorrectly set to a date period of "week" when it's monthly data.

But by a happy coincidence, their data is changing from monthly to weekly in a current story. So I'm not going to "fix" it only to have to change it back again later today.