Note we usually cannot make it simple because officer assignments are
usually assigned to both a poll and a booth, and on a certain date.
However, in the few cases where the booth nor the date don't matter, we
can make the code a bit easier to read.
While in theory we wouldn't need to use the `transient` nor the
`after(:create)` because there's already a `has_many :through`
association with followers, Factory Bot / ActiveRecord don't
automatically associate the followable, resulting in an invalid record
exception.
