Only display ticks on y-axis of graph when showing data

To prevent case where whole-number graphs were shwoing [0, 0.5, 1] as y axis values, which is incongruous with integer values.
Merge pull request #506 from alphagov/bugfix/y-axis-ticks

Only display ticks on y-axis of graph when showing data
Return zero values fr completion when provided with zero input

Completion collection was incorrectly parsing 0 as "no data", which caused errors when datasets contain actual zero values.
Merge pull request #507 from alphagov/bugfix/completion-numbers-zero-values

Return zero values for completion when provided with zero input
Move content dashboards out of /experimental/
Add slug to visitors and pageviews
Fixes from bug bash
Fix capitalization @alexmuller
Change dashboard-type to 'content' and published to 'true'
Move tagline out to a top-level property
Update cheapseats SHA