Simplify EditionsController

Because of the way the form fields are now rendered, we no longer need
to pre-build all the join models ahead of time in the controller action.
Consolidate EditionOrganisation filters for :new

Why have two filters when you can have one?
More before_filter simplications

Instead of duplicating the before_filter across multiple controllers,
we can include it in the base class and only execute it if appropriate,
i.e. when the edition type supports images