Reuse method for querying for a date range

It's also used for the other LPA modules. Using the same so we query for
the same week.
Calculate total weeks based on first and last event
Only when both start and done are missing, treat as missing data

Otherwise, if one is present and zero, data is available and should plot
data in the graph.
Remove unused functions
Removed 'FCO' reference from html title tag
Removed 'Vehicle licensing' reference from html title tag
Merge pull request #216 from alphagov/page_title_copy_changes_57987954

Page title copy changes 57987954
Remove duplication when creating volumetric series
Merge pull request #215 from alphagov/fix-help-usage-query

Reuse method for querying for a date range
Merge branch 'master' into fix-missing-journey-data
Merge branch 'fix-missing-journey-data' into lpa-completion-rate