We've now created two tables, each of which allows us to answer an important question about follow relationships: first, whom does a user follow; and second, who follows a user. Now let's establish some follow relationships.

For now, let's have alice follow a couple of other users, bob and carol:

INSERT INTO "user_outbound_follows"
  ("follower_username", "followed_username")
VALUES ('alice', 'bob');
INSERT INTO "user_inbound_follows"
  ("followed_username", "follower_username")
VALUES ('bob', 'alice');
INSERT INTO "user_outbound_follows"
  ("follower_username", "followed_username")
VALUES ('alice', 'carol');
INSERT INTO "user_inbound_follows"
  ("followed_username", "follower_username")
VALUES ('carol', 'alice');

For each follow relationship, we have to insert two rows: one in the user_outbound_follows table to store the relationship from the perspective of the follower, and one in the user_inbound_follows table to store the relationship from the perspective of...