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
Add test for percentage normalisation
Add high volume transaction dashboard type to allowed list
Add content dashboard type to allowed list
Merge pull request #416 from pwaller/percentage-normalisation

Add 'normalisation' parameter to 'percent' formatter
Merge pull request #417 from alphagov/feature/dashboard-types-enum

Feature/dashboard types enum
Added transaction explorer dashboard generator.
Added high volume transaction dashboards.
Fixed description fields for dashboard.

Added notes sections.
Added digital take-up module generator.
Added digital-take-up for high volume services.
Added percentage format to digital-takeup
Fix issue caught by the schema validation.

Only add a related link if one is available in the response.
Fixed weird character issue
Trim whitespace
Changed dashboard-type as per @lennym request.
Updated properties on digital_takeup as per @annaps request.