Refactor Person/Friendship relationships to be more intuitive

PR #5210 added a Friendship model to illustrate a bug, but in doing so
created a confusing structure because both belongs_to declarations in
Friendship referred to the same side of the join. The new structure
maintains the integrity of the bug test while changing the follower
relationship to be more useful for other testing.
Merge pull request #9747 from macksmind/refactor_friend_follower

Refactor Person/Friendship relationships to be more intuitive