Use correct collection lookup for graph periods

collection.options.period is always undefined, so this would return "week" irrespective of the actual configured period.

Update cheapseats pointer to include tests that this caused to fail.
Merge pull request #672 from alphagov/bugfix/single-timeseries-durations

Use correct collection lookup for graph periods
Fix group_by params in site-activity dashboards that got broken in migration

A bug in the migrator caused the group_by params to be overwritten with "eventCategory" instead of their correct values