Add grey baseline to grouped timeseries graphs

Position relative is required to ensure that the graph lines appear in front of the baseline when ~0
Add tick lines to graph y-axis

Except for sparklines.
Add new graph styles to availability module

Inherit styles from single-decker-graph to avoid loads of duplication.
Make availability graphs work like other graphs

Graphs should show the latest value when not being hovered, not a long term mean.

This means we can delete basically all the code, and just inherit from volumetrics/number.
Move live service usage number to the right
Output no data when appropriate in avilability modules
Ensure hourly data is formatted appropriately
Remove tests for deleted code
Merge pull request #699 from alphagov/enhancement/grouped-timeseries-consistency

Graph consistency changes