Fix topics#show and other refactorings.

The main thrust of this commit is to fix the topics#show page so that
it correctly lists statistical publications as apposed to statistical
data sets, mirroring organisations#show. As part of the work, I
thought I'd tidy up some stuff that's been bothering me for a while.
In particular, the use of assocaitions in place of scopes.

Associations are being used in a few places where scopes are more
appropriate. They are both simpler and more re-usable - by adding
scopes to the Edition model, they can be used by anything that has an
:editions association. This reduces the duplication between models.

Also, the state machine gives us scopes for edition states for free,
so we don't need to define associations for stuff like
published_polices. Associations should only ever be used if you intend
to manipulate the association by adding/removing from it.

I've also killed off some unused class methods on Classification.
Merge pull request #911 from alphagov/fix-topics-show

Fix topics#show and other refactorings.