AR#readonly behaviour has changed

Rails 4 changes the behaviour of ActiveRecord such that a model that
is marked as "readonly" can no longer be destroyed (see
https://github.com/iGEL/rails/commit/a8b370f5c2fed87001e363d18ce60a47c1748885).
This prevents EditionRelations and ClassificationRelations from being
destroyed. Looking back through the git history, it looks like this
was originally put in as a way to prevent these join models from being
modified once they were created. I can't think of any good reason why
we should be enforcing this at the model level so I'm simply removing
the readonly designation from the model.
Don't rely on transient behaviour of factories/AR

Instead, the test should replicate the way that groups are created on
collections in real-world app usage.
No more need for distinct optoin on count