Break RootController#index into a set of smaller methods

These methods are far from ideal and mix side-effects (setting instance vars)
with return values in an awkward way. The intent of these changes is to begin
breaking the main method into smaller pieces to make it a little clearer how
it's constructed and how we might move to a cleaner implementation.
Tidy up broken out RootController#index methods

Broken up a little more, and no longer setting instance variables
all over the place. I think this is probably as far as we want to
go without opening up the question of breaking this out into filter
classes and/or revisiting how the AdminRootPresenter works.

Conflicts:
	app/controllers/admin/root_controller.rb
Prefer Object#present? over !Object#blank? for clarity

Conflicts:
	app/controllers/admin/root_controller.rb
Move creation of list parameter to appropriate place

It couldn't work where it was as it didn't have an edition to look at.
This method is now returning three values, which is a smell. Going further
seems to involve a larger refactoring that I think ought to be done as
a follow-on from this branch.
Convert stray @user_filter to user_filter
Fix param name
Merge pull request #88 from alphagov/break-up-main-index-code

Break up main index code