Add devise_token_auth for users

This add API authentication support. As of now, we are using a custom fork of `devise_token_auth` as they have a baked in dependency on ActiveRecord. However, I am in talks with the community on removing that dependency (as it really should not be there in the first place :smile: ).

Here's the relevant thread:  https://github.com/lynndylanhurley/devise_token_auth/issues/72