Convert CHANGELOG-EE to Markdown

All this does is convert the version sections into headers. The list
items shouldn't really be indented by two spaces, but it makes no
difference to the rendering and this way we retain authorship history
for the actual changes.

[ci skip]

Related to https://gitlab.com/gitlab-org/release-tools/merge_requests/29

.gitattributes

 CHANGELOG merge=union
-CHANGELOG-EE merge=union
+CHANGELOG-EE.md merge=union
 *.js.es6 gitlab-language=javascript