feat: Add a styled 404 page, rendered on the back end.

* Add consolidate middleware to handle the interface between express and handlebars
* Add a middleware to respond to 404s
* Add a branch to the 't' handlebars helper to deal with 't' when invoked in templates written using res.render
* Update grunt tasks to manage dist mode server rendered templates.

BREAKING CHANGE:
The production configuration script must add a new option to its config :
```
   page_template_subdirectory: 'dist'
```

issue #554

.
refactor(i18n): use dependency injection to pass i18n instance around
Merge pull request #589 from mozilla/issue-554-four-oh-four-style

feat: Add a styled 404 page, rendered and translated on the back end.