Devise setup

I set up user authentication using Devise, mostly following the example in RailsApps tutorial (https://tutorials.railsapps.org/tutorials/devise-quickstart#how-rails-composer-sets-up-an-application-with-devise) -- sorry, may be paywall protected. The example code isn't paywall-protected though: https://github.com/RailsApps/rails-devise .

Next steps planned: authorization using Pundit and Rolify.