Introduce capybara and first feature spec (#1801)

This commit introduces Capybara and the first feature spec.

I focused on coverage for log in for the first feature spec because that would
have prevented 624a9a7136 causing #1236.

Gemfile

@@ -70,6 +70,7 @@ group :development, :test do
 end
 
 group :test do
+  gem 'capybara'
   gem 'faker'
   gem 'rails-controller-testing'
   gem 'rspec-sidekiq'