Update Grafana Dashboard with MoM Churn

[coreymjames]

Note: This needs more clarity w/r/t exactly how we are measuring churn

[juliangruber]

@coreymjames reassigning to you since this issue first needs a churn definition

[patrickwoodhead]

If we use the definitions:

• New users: those who have submitted a measurement in the last month (or other epoch) but never before
• Regained users: those who have submitted a measurement in the last month (or other epoch) but not the month (epoch) before
• Maintained users: those who have submitted a measurement in each of the last two months (epochs)
• Lost users: Those who submitted a measurement in the month (epoch) before last but not the most recent month (epoch)

Then churn = lost users / (maintained users + lost users)