set proper account in preview iframe

fixes CNVS-23657

test plan:
  - make a parent theme and appluy
  - make a subaccount theme and apply
  - go back to the editor for the parent
    - it starts off with its current theme
  - go back to the editor for the subaccount
    - it starts off with its current theme
  - changes to both apply correctly

Change-Id: I77d306be0fe34e0fcb6305bd799240ebb45072f0
Reviewed-on: https://gerrit.instructure.com/64032
Tested-by: Jenkins
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>

Author: Michael Nomitch

app/controllers/info_controller.rb

@@ -55,4 +55,9 @@ def browserconfig
     expires_in 10.minutes, public: true
   end
 
+  def theme_preview
+    if params[:account_id]
+      @account = Account.find_cached(params[:account_id])
+    end
+  end
 end

app/jsx/theme_editor/ThemeEditor.jsx

@@ -436,7 +436,7 @@ define([
                     </button>
                   </div>
                 : null }
-                <iframe id="previewIframe" ref="previewIframe" src="/theme-preview/?editing_brand_config=1" />
+                <iframe id="previewIframe" ref="previewIframe" src={"/accounts/"+this.props.accountID+"/theme-preview/?editing_brand_config=1"} />
               </div>
 
             </div>

config/routes.rb

@@ -745,7 +745,7 @@
   get 'dashboard-sidebar' => 'users#dashboard_sidebar', as: :dashboard_sidebar
   post 'users/toggle_recent_activity_dashboard' => 'users#toggle_recent_activity_dashboard'
   get 'styleguide' => 'info#styleguide'
-  get 'theme-preview' => 'info#theme_preview'
+  get 'accounts/:account_id/theme-preview' => 'info#theme_preview'
   get 'old_styleguide' => 'info#old_styleguide'
   root to: 'users#user_dashboard', as: 'root', via: :get
   # backwards compatibility with the old /dashboard url