Fix Contribution Analytics and move test to Rspec.

Apparently Spinach tests don't fail when there are JS errors, which is why this didn't get caught earlier. This should future-proof it.

Fix #871.

app/views/groups/analytics/show.html.haml

 - page_title "Contribution Analytics"
 - header_title group_title(@group, "Contribution Analytics", group_analytics_path(@group))
+- content_for :page_specific_javascripts do
+  = page_specific_javascript_tag('lib/chart.js')
 
 .gray-content-block
   .pull-right

features/group/analytics.feature

-Feature: Group Analytics
-  Background:
-    Given I sign in as "John Doe"
-    And "John Doe" is owner of group "Owned"
-
-  Scenario: I should see group "Owned" analytics page
-    When I visit group "Owned" page
-    And I click on group analytics
-    Then I should see group analytics page

features/steps/group/analytics.rb

-class Spinach::Features::GroupAnalytics < Spinach::FeatureSteps
-  include SharedAuthentication
-  include SharedPaths
-  include SharedGroup
-  include SharedUser
-
-  step 'I click on group analytics' do
-    click_link 'Contribution Analytics'
-  end
-
-  step 'I should see group analytics page' do
-    expect(page).to have_content "Contribution analytics for issues, merge requests and push"
-  end
-end

spec/features/groups/analytics.rb

+require 'spec_helper'
+
+feature 'Groups > Contribution Analytics', js: true, feature: true do
+  let(:user) { create(:user) }
+  let(:group) { create(:group) }
+  let(:empty_project) { create(:empty_project, namespace: group) }
+
+  before do
+    group.add_owner(user)
+    login_with(user)
+  end
+
+  describe 'visit Contribution Analytics page for group' do
+    it 'displays Contribution Analytics' do
+      visit group_path(group)
+
+      click_link 'Contribution Analytics'
+
+      expect(page).to have_content "Contribution analytics for issues, merge requests and push"
+    end
+  end
+end