Especially with all the "list" commands, we now see that we always have
to put some data into the instance. This is cumbersome, especially as we
get to the track commands. Therefore, I added a method to put some data
into the instance -- that also helps to ensure that we properly cascade
things if we delete a user, for example.

Currently, this method only adds two users and two tracks, but I plan to
also add comments and friendship associations.

Further, we can think about re-writing the playwright tests to use this
population as well, instead of the playwright_helper one.