Standardise valueAttr across all collections

Grouped timeseries was using valueAttr and completion valueAttribute.
Switch everything to using valueAttr.
Tidy completion rate params

We know all of these values will exist as we set them in the
initializer, no need to pull them from the options.
Let comepletion rate collection be self supporting

The completion rate graph is now its own thing and doesn't extend the
completion collection. It also now lets backdrop do all the logic to
work out time buckets.

Updated the stagecraft stubs to include the actuall value attributes
which will come back from backdrop rather than trying to guess them.