Add statistics heartbeat to be used in "your samarbeid" program

This adds a small service which collects the (daily) anonymized usage statistics for a single samarbeid instance.

This is intended to be used as described in the conditions for the "samarbeid für Euer Team" program.

An example of how a CSV report (for several days) may look like can be found under test/fixtures/files/example_stats_heartbeat.csv