Handle edge cases in sign_in test helper

The sign_in helper requires FactoryGirl to create a user. FactoryGirl isn't a
dependency of the Gem and we wouldn't want it to be because we don't want it
loaded in production. Raise a more helpful error if someone tries to use the
`sign_in` test helper without having FactoryGirl.

Additionally remove the hard-coded `:user` factory in favor of a factory named
for whatever the configred user model is.