Fix reference links in dashboard activity and ATOM feeds.

CHANGELOG

app/helpers/events_helper.rb

app/helpers/gitlab_markdown_helper.rb

app/views/events/_event_issue.atom.haml

app/views/events/_event_merge_request.atom.haml

app/views/events/_event_note.atom.haml

app/views/events/_event_push.atom.haml

app/views/events/event/_note.html.haml

lib/gitlab/markdown.rb

lib/redcarpet/render/gitlab_html.rb

spec/helpers/gitlab_markdown_helper_spec.rb