Stop using mean() when displaying volumetrics data

We now get the last model with data from the collection.
Include a set of default type styles…

…so you can create a page containing standard elements (eg p, h1, h2,
h3 etc) and it typesets nicely
Remove .performance-platform-outer

.performance-platform-outer wasn’t doing anything we couldn’t do with
padding on the parent element
$govuk-blue on border-top and border-bottom

Replace PP $primary-colour with $govuk-blue because we are one GOV.UK
Reduce size of impact number on desktop
Merge pull request #649 from alphagov/typography

CSS refactor part I
Fixed bug in getValue that would try and do a .get on an undefined.

If lastDefined can't find a last defined model in the collection it will do a
pop() on an empty array returning 'undefined' this means we need to test that we
have a valid model before doing anything with it. if we don't we should call
formatValue with null (to get no data)

Added tests for this use case.
Merge pull request #677 from alphagov/feature/line-labels

Stop using mean() when displaying volumetrics data
Get the correct formatPeriod off the collection.
Fix start dates following migration

The migration of dataset parameters caused an hour of timezone-ness to get added to the dates. This resulted in the first data point of the sets to return "no-data" (since I assume the monthly data is all in the database at a timestamp of midnight on the first of the month)
Fix typo in 'Pay register death abroad' dashboard

https://govuk.zendesk.com/tickets/780439
Merge pull request #682 from alphagov/bugfix-fco-death-abroad-text

Fix typo in 'Pay register death abroad' dashboard
Test the rendering of summary time periods
Prevent stacked graphs from selecting when no model is selected

Equivalent to 3240392c769ac522dbc15eae42a71393d32e434f but for stacks as well as lines.
Fix chrome bug where border top was disappearing after sort
Add tests for stack selection on mousemove
Merge pull request #683 from alphagov/bugfix/stacked-graph-hover

Prevent stacked graphs from selecting when no model is selected
Merge pull request #681 from alphagov/bugfix/timezone-weirdness

Fix start dates following migration
Merge pull request #684 from alphagov/bugfix/sortable-tables-border

Fix chrome bug where border top was disappearing after sort
Merge pull request #680 from alphagov/bugfix/periods

Get the correct formatPeriod from the collection