Add stub data for "time taken to complete transaction"

Correctly-formatted data is not yet in production (it's currently in
seconds rather than milliseconds) so it may be easiest to test
against this data.
Extend single-timeseries module with global format options

Create a module-wide set of format config options, so that
all elements of the module can refer to these options.

Pass these through the module and the view.
Extend duration formatters to return minutes and seconds

If passed 1234500ms and the 'm' unit option, return '20m 35s'.

The existing minute formatter isn't used anywhere in the app,
so just replace it.

This display option is for the time-taken-to-complete graph,
where data is most useful displayed in minutes and seconds.
Use a time scale in graph.js for duration data

Extend graph.js to use a time scale for data in duration format.
Set the domain of the scale using D3's .nice() function, not
by supplying an explicit domain and tick value list.

Supply the appropriate time interval to .nice() by looking
at the maximum value of the data.
Format submissions-graph y-axis for duration data

Use D3 time formats to display y-axis ticks nicely on our duration
graphs. Update tests.
Add time-taken-to-complete graph on Carer's Allowance
Set default formatting options at module level

Allow axes to over-ride individually if required.

Also update unit formatter for "time taken to complete" data
from "s" to "m", now that we are converting data into milliseconds
on the back-end.
Merge pull request #653 from alphagov/feature/time-taken-to-complete-graph

Feature/time taken to complete graph