Details/Summary element in GFM needs some UI polish

1. There should be some space at the bottom of the details element, so it's not so tight against whatever comes next.
2. We should parse markdown (in the gif below, there's an unordered list) inside the details element, see https://gitlab.com/gitlab-org/gitlab-ce/issues/30846
3. The focus on the summary toggle shouldn't persist -- it looks weird

See it live: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12103#note_32134869