Remove sign up link from sign in flash error

There is a configuration option that allows for sign up to easily be disabled.
With this enabled, the `sign_up_path` route helper is not defined. This causes
a 500 when a user login fails because the default I18n message uses
`sign_up_path` for interpolation.

I opted to remove the interpolation, and thus the sign up link, from the
default sign in failure message. This is perhaps somewhat less helpful, but it
makes for a working out of the box experience for people who disable sign up.

An alternative would be to add another flash message specifically for this
case and add conditionals to `DefaultSignInGuard` (or a helper object) to
handle it. I felt that the increased complexity of that solution wasn't worth
it.

This addresses #400.