Remove associations from ShiftEvent & Shift factories #33

* `Taxonomy 9`
* `Taxonomy 10`
Would actually sort in the opposite order because `1` is before `9` in
a string. Switching to letters ensures that sorting by name (as the
`ExportService` does) will keep them in the order they were created in.
Technically, this could still cause a problem if the sequence gets
bigger by one letter:
* `Taxonomy ZZZZ`
* `Taxonomy AAAAA`
But that would require a little under half a million of these to be
generated in a single test run, so it's unlikely.

These associations create unnecessary data in the database. For
example, the `shift` association in the `ShiftEvent` factory creates an
additional `shift_event`, which means creating a `shift_event` from a
factory actually creates 2. In addition, every `shift_event` creates
one `event`. That means creating an `event` from a factory creates 3
events:
- one from the factory,
- one from the shift event,
- and one from the shift which creates an additional shift event
It seems like this was done to construct shifts, events, and
shift_events more easily in tests. For example, `ShiftEvent` does not
directly have an organization; `Event` does. But the factories were set
up so you could assign those properties and the automatically created
associations would use them.
This commit removes those extra associations now that the tests all use
`event` factories.