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