Pin Carer's completion rate module to 19 May

Instead of displaying data going back 9 weeks, just display back to the 19th of May because that's when we started collecting data with the `thank-you` label.

This change should be reverted in a few weeks once there is enough data in place to make a decent-looking graph. [A chore has been created for that](https://www.pivotaltracker.com/story/show/72963084).