Flexible filled line graph

Extend the completion_rate and completion_numbers modules to support flexible date periods, axis periods, durations, and flexible value parameters. Pivotal story: https://www.pivotaltracker.com/s/projects/911874/stories/65892596

Also rename "start" and "end" related parameters (technical debt left over from when this module was only used for transactions) to "denominator" and "numerator" respectively. Pivotal story: https://www.pivotaltracker.com/s/projects/911874/stories/63460908

Note that I haven't updated `module-schema.json` with the schema changes, as this file currently looks problematic to me in a number of ways - will discuss and update this separately.