Fix ProjectCacheWorker for plain READMEs

What does this MR do?

The ProjectCacheWorker refreshes cache periodically, but it runs outside Rails context. So include the ActionView helpers so the content_tag method is available.

Are there points in the code the reviewer needs to double check?

I added include ActionView::Context because, I ran into this error:

NoMethodError: undefined method 'output_buffer=' for

See http://stackoverflow.com/a/7562194/89376

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes gitlab-org/gitlab-ce#32540

app/helpers/markup_helper.rb

 require 'nokogiri'
 module MarkupHelper
+  include ActionView::Helpers::TagHelper
+  include ActionView::Context
+
   def plain?(filename)
     Gitlab::MarkupHelper.plain?(filename)
   end