Let graphs have different period for data and axis

In some situations you want a more detailed graph line than the axis
which is used. Add an option to the stagecraft response to allow the
axis period to be set.

- Removed code which previously calculated axis ticks and use the native
  d3 maths
- Pass an 'axis-period' option through from the stagecraft response to
  the collection options
- Use the 'axisPeriod' when selecting the formatting for the axis
- On hour and day axis default to only showing 4 ticks so they don't
  visually run into each other
- Update the site-activity graphs to show a years worth of data in weeks
  with month axis labels

This isn't quite dynamic as this story describes: https://www.pivotaltracker.com/story/show/63526068 the logic for that may possibly be very complex and this is MVP for the current requirements of these two graphs.