Default values in journey-series collection

When backdrop does not have any hits at all for one of the steps in a journey collection it returns no data point, which means the graph bar is rendered with a "no data" label.

If we have data for some of the steps, then assume that the value for the missing step should be zero, rather than "no data".
Merge pull request #413 from alphagov/bugfix/journey-module-missing-data

Default values in journey-series collection
Refactor get tagline script so it doesn't require modules to have a dashboard type of other

We want to be able to have a configured tagline without forcing a specific dashboard type, so we add a fallback of just having a tagline property on the model.

Ideally we'd also get rid of the "other" stuff, but it stays for backwards compatibility.
Add support for high-volume-transaction option as dashboard-type
Merge pull request #414 from alphagov/refactor/get-tagline

Refactor/get tagline
Update experimental site-activity dashboards

This update includes many small fixes requested by the designers,
mostly language fixes and small functionality fixes.
Merge pull request #411 from pwaller/update-content-dashboards

Update experimental site-activity dashboards
Add "normalisation" option to "percent" formatter

This is so that values represented as 0-100 in backdrop can be correctly
formatted.
Use 'normalisation' to correctly format percentage

... in content dashboards