Move devise controllers from lib to app

[bbuchalter]

There are four controllers which inherit from Devise instead of Spree:

• UserConfirmations
• UserPasswords
• UserRegistrations
• UserSessions

These controllers are required for solidus_auth_devise to work properly, but can be excluded from loading if the application using the gem does not use spree_frontend. Therefore, we should move these Devise-specific controllers out of a conditional load path.

[bbuchalter]

After talking with @adammathys we agreed that it would be better to have the devise front-specific routes not loaded if spree_frontend is not present. This ensures applications which implement their own frontend have maximum flexibility and total control. This PR would actually restrict their ability to control their own front end and creates an implicit assumption that views must be present in certain paths.