Add cached containers for organisations
Add caching edition organisation select fields

This also pulls out the generation of the selects from the form builder
helpers to plain-old vanilla form helpers. I hope to simplfy these and
the code in the controllers to pre-load the organisation join models
in the controllers in a later commit
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
Simplify and rename helper method

The new name better reflect what the method is returning. The method was
also over-complicated:

* It had a boolean flag switching whether it returned a link or just
plain text, but there is nowhere in the app that uses it to generate a
link.
* No `current?` role appointment will ever have an ended_at timestamp,
so the ternary operator check is redundant.
* There is no HTML in the text, so marking it html_safe is unnecessary.
Typos and omissions