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
Use query period for graph callout formatting

Rather than using the config to set the formatting period use the period
from the query. It then isn't coupled to the same config as the axis are
using.

- Remove getTick as it is no longer used anwhere
- Update tests based on the new d3 date logic
Allow array of arguments for axis config options.

This is to allow the "ticks" option to pass more than one argument if required.
Use UTC for D3 ticks, and start dates for ticks.

* Use start dates, not end dates, as axis tick labels: this is better logically,
  because if we are showing data from Jan 1st to Feb 1st, it is data for Jan,
  not Feb. Similarly, if we are showing data from Feb 1st to Feb 7th, it is data
  for the week of Feb 1st, not the week of Feb 7th. This has been okayed with
  design (Henry).
* Update availability collections to set timestamp to start_date, not end_date,
  because of the above change to use start dates everywhere.
* Use D3's native time intervals to set our axis ticks, and ensure we're using
  UTC D3 methods, to play nicely with our UTC tick formatting.
  * Update hourly graph to show ticks every six hours, rather than using four
    hard-coded ticks.
  * Update daily graph to show ticks on a Monday, to match the start dates
    for our data.
  * Ditto for weekly graphs.
  * Update monthly graph to use D3's native UTC month interval.
* Update tooltip formatting for quarterly graphs, as this was removed in
  previous commit.
Include tests for getConfigNames in stacked + line

There were no tests for LineGraph, so we have added some and tested its
functionality. The tests for StackedGraph were functionally fine but
their descriptions were a little confusing.
Merge pull request #200 from alphagov/axis-times-fixed

Let graphs have different period for data and axis