Copy code as GFM from diffs, blobs and GFM code blocks
Followup to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8597
@jschatz1 @smcgivern Please review the frontend and backend!
The interesting new bits of spec/features/copy_as_gfm_spec.rb
start at line 435, the rest was just indented one extra step.
/cc @rspeicher @victorwu