Refactor model class names
Remove vanity URL prefixes

This change simplifies both the URLs and the code, but it does mean
that both generated short URLs and their custom aliases are in the
same namespace, which could cause collisions.
Add additional error condition tests
Simplify invalid decode warning text

The string is already displayed in the error text, so there's no
reason to duplicate it.
Further refactor model names
Add alias to admin display and search
Handle alias as lowercase, not uppercase