Implement monitoring / sanity-checking

[philbooth]

It's not clear to me how long this repo has left in light of the Amplitude stuff, but if it's a while one thing I'd like to implement is monitoring and automated sanity-checking.

Kind of thing I have in mind, to be run daily post-import:

• check MIN and MAX timestamps for each data set
• check relative daily event count is within reasonable limits for each data set
• check relative numbers for certain event types seem reasonable
• check that joining flow and activity events on uid produces expected number of results
• check that metadata is set on expected number of flows for the most recent day

[philbooth]

Done in https://sql.telemetry.mozilla.org/dashboard/fxa-sanity-checks.