Add failing acceptance test for ERB-rendered 404 page
Render 404 template with ERB and dummy site attributes
Add failing acceptance test for site-specific 404 page
Add "site" attribute to sites in acceptance tests

This is essentially a slug, used to uniquely identify the site for
configuration purposes.
Clean test database before & during tests
Stop manually cleaning the hosts table
Force creation of minitrue site before all acceptance tests
Use different attributes for different sites
Add Organisation model

The information for the 404 template actually lives on the organisation,
so we need to be able to follow this association from the site.
Read site attributes from database
Copy over assets from alphagov/redirector
Serve assets with Rack::Static
Move assets into public/ subdirectories
Update asset URLs in 404 template and stylesheet
Provide per-organisation CSS class to 404 template
Reorganise code to create binding from within a dedicated method

This helps to avoid the binding being polluted by local variable
assignments (e.g. host) from inside #call.
Include host and tna_timestamp in 404 page
Set Content-Type correctly for 404 pages