spec/features/markdown_spec.rb takes at least 4 minutes to run so it should be split

spec/features/markdown_spec.rb takes 4 minutes to run locally, while it takes 5 minutes on the CI for MySQL (https://gitlab.com/gitlab-org/gitlab-ce/builds/17458144), and 15 minutes on the CI for PG (https://gitlab.com/gitlab-org/gitlab-ce/builds/17463191)! The difference in PG is probably due to resources limitations since all the specs take longer there... The runner was the same, though.

This spec should split into several spec files in order to be better spread across test nodes.