Make a govuk_template asset directory

...and move the appropriate assets there for easier updates to template
assets.
Add error handling for 404s and 500s
Add response locals pageTitle

This gets around the need to have an object called ```transaction```
when rendering the response.
Update the root redirect to /start
Merge pull request #15 from alphagov/error_handling

Add custom error pages.
Merge pull request #14 from alphagov/make_govuk_template_asset_dir

Make a govuk_template asset directory
Merge pull request #16 from alphagov/update_root_redirects

Update root redirects to /start
Merge branch 'master' into winazure_preview_deployment