Use Gitlab::Markdown.render with :pipeline option rather than different methods

app/helpers/gitlab_markdown_helper.rb

app/views/events/_commit.html.haml

app/views/projects/commit/_commit_box.html.haml

app/views/projects/commits/_commit.html.haml

app/views/projects/issues/show.html.haml

app/views/projects/merge_requests/show/_mr_box.html.haml

app/views/projects/merge_requests/widget/_open.html.haml

app/views/projects/milestones/show.html.haml

app/views/projects/repositories/_feed.html.haml

lib/gitlab/markdown.rb

lib/gitlab/markdown/markdown_filter.rb

lib/gitlab/markdown/relative_link_filter.rb

lib/gitlab/markdown/sanitization_filter.rb

lib/gitlab/reference_extractor.rb