Add feature flag to authentication feature
Make Sign in, My Account and Register be links and not buttons
Add authentication routes

We can replace these with devise in due course, but for now we get:

- Deliberate specs to guide us when the routing changes
- A slightly more elegant solution for resolving authentication routes in development
- Removes URL duplication in views
- Removes need to change the views when the application gains the ability handle the routes itself
- Lets the router take care of locale scope
- Lets us continue to use the locale concern to set default routing options

cc/ @stevenwilkin @alexwllms