DEV: makes discourse-styleguide core (styleguide) (#10847)

This plugin is only useful for developers, however, making it core allows us to centralize any component modification in one commit.

This integration also adds a new site_setting: `styleguide_admin_only` which allows to enable a styleguide on a live site while restricting visibility to admins only.

By default, styleguide is disabled.

plugins/styleguide/spec/integration/access_spec.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'SiteSetting.styleguide_admin_only' do
+  before do
+    SiteSetting.styleguide_enabled = true
+  end
+
+  context 'styleguide is admin only' do
+    before do
+      SiteSetting.styleguide_admin_only = true
+    end
+
+    context 'user is admin' do
+      before do
+        sign_in(Fabricate(:admin))
+      end
+
+      it 'shows the styleguide' do
+        get '/styleguide'
+        expect(response.status).to eq(200)
+      end
+    end
+
+    context 'user is not admin' do
+      before do
+        sign_in(Fabricate(:user))
+      end
+
+      it 'doesn’t allow access' do
+        get '/styleguide'
+        expect(response.status).to eq(403)
+      end
+    end
+  end
+end
+
+describe 'SiteSetting.styleguide_enabled' do
+  before do
+    sign_in(Fabricate(:admin))
+  end
+
+  context 'style is enabled' do
+    before do
+      SiteSetting.styleguide_enabled = true
+    end
+
+    it 'shows the styleguide' do
+      get '/styleguide'
+      expect(response.status).to eq(200)
+    end
+  end
+
+  context 'styleguide is disabled' do
+    before do
+      SiteSetting.styleguide_enabled = false
+    end
+
+    it 'returns a page not found' do
+      get '/styleguide'
+      expect(response.status).to eq(404)
+    end
+  end
+end