Move everything out of the admin namespace

This is a hangover from the earliest days of this app when it had
quite a few other responsibilities. It's been redundant for at
least 18 months and is now extra noise we don't need.
Add redirects for old /admin routes

It may be possible to do this in a single route but I wanted
to make it clear that we support both and avoid too much regexp
syntax.

There aren't any tests for this. It's pretty straightforward
rails code but I'd be happy to add a few if there are strong
feelings on the matter.
Remove BaseController and give ApplicationController control

With the admin namespace gone the BaseController is an unnecessary
layer of indirection.
Change AdminRootPresenter to PrimaryListingPresenter

The word 'admin' isn't needed here and this name feels a bit clearer.
Simplify redirect to remove /admin prefix from paths

Adds a test and simplifies to a single route declaration
Merge pull request #156 from alphagov/remove-admin-namespace

Remove admin namespace