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.

spec/features/log_in_spec.rb

@@ -0,0 +1,16 @@
+require "rails_helper"
+
+feature "Log in" do
+  scenario "A valid email and password user is able to log in" do
+    email = "test@example.com"
+    password = "password"
+    Fabricate(:user, email: email, password: password)
+
+    visit new_user_session_path
+    fill_in "user_email", with: email
+    fill_in "user_password", with: password
+    click_on "Log in"
+
+    expect(page).to have_css "div.app-holder[data-react-class=Mastodon]"
+  end
+end

spec/rails_helper.rb

@@ -7,6 +7,7 @@ require 'spec_helper'
 require 'rspec/rails'
 require 'webmock/rspec'
 require 'paperclip/matchers'
+require 'capybara/rspec'
 
 Dir[Rails.root.join('spec/support/**/*.rb')].each { |f| require f }
 
@@ -24,6 +25,11 @@ RSpec.configure do |config|
   config.include Devise::Test::ControllerHelpers, type: :controller
   config.include Devise::TestHelpers, type: :view
   config.include Paperclip::Shoulda::Matchers
+
+  config.before :each, type: :feature do
+    https = ENV['LOCAL_HTTPS'] == 'true'
+    Capybara.app_host = "http#{https ? 's' : ''}://#{ENV.fetch('LOCAL_DOMAIN')}"
+  end
 end
 
 RSpec::Sidekiq.configure do |config|