Remove ymin config

this wasn't really actually doing anything useful. It was only used in sparklines, which were only used for real-time usage graphs. These always ended up with a min y value of 0, in the early hours of the morning, and it seems that we would always want these to be scaled from zero anyway.
Fix graph tests so they test what they say they do

Tests for "week" period were being config'ed with "day"
Also add tests for other applicable periods
Move `calculateLinearTicks` method from View into Graph

This is only relevant to graphs, and was only ever called from inside the Graph view.
Move methods from "overlay" config to graph as defaults

The default config for all graphs was overlay, so have the methods from that as default methods on a Graph instance, instead of extending at render-time.
Where those methods are not desired on child classes, extend in those isntances.
Remove last traces of overlay config
Add tests for new JourneyGraph calcYScale method
Extend line-label for stacked graphs instead of applying config madness
Move stacked graph functionality to stacked-graph view

Inherited from there for modules that require "stacked" functionality.
Prevent stacked graph from overwriting valueAttr option
Remove getConfigNames/applyConfig completely

Last remainin uses for these were to do period-based calculations for axes. Instead we replace the getConfigNames with a getPeriod method, and use this for period-dependent functionality.
Extend, rather than reimplement slightly different percentage graphs
Remove console log
Ensure model is passed to multi_line_stat view

It needs to be able to load the period from config
Return minvaue functionality to sparklines

It makes some render a lot more nicely.
Fix sparkline lower bound method

It was returning 1 as a fallback as a result of a copy-paste fail. The minimum/default value should actually be 0.
Merge pull request #490 from alphagov/refactor/graph-configs

Refactor/graph configs