Can't remove crypted_password or salt from User model

[joshbuker]

NoMethodError:
       undefined method `crypted_password' for #<User:0x007fac4838f918>
       Did you mean?  encrypt_password
     # /Users/jbuker/.rvm/gems/ruby-2.3.1/gems/sorcery-0.9.1/lib/sorcery/model.rb:146:in `external?'
     # /Users/jbuker/.rvm/gems/ruby-2.3.1/gems/sorcery-0.9.1/lib/sorcery/model/submodules/user_activation.rb:127:in `send_activation_success_email?'
     # /Users/jbuker/.rvm/gems/ruby-2.3.1/gems/sorcery-0.9.1/lib/sorcery/model/submodules/user_activation.rb:111:in `activate!'

For my particular use-case, I need users that only authenticate with external providers. Currently it works, but if I try to remove the crypted_password and/or salt fields from User, it gives this error. I tried setting the config to use nil or '' instead, but that also gives errors.

When we figure out how to go forward with the modularization, we should have the option to exclude the usual crypted_password setup.

[joshbuker]

This issue (#12) should be considered if we do try addressing: #217