Merge branch 'fix-xmlns-namespacing-atom-feeds' into 'master'

Fix xmlns:media namespacing for atom feeds

A recent commit gitlab-org/gitlab-ce@d31d711a70779132ea43387f93eb4ccc1e472761 introduced a bug that broke four of the atom feeds. The namespace for xmlns:media was not defined properly. This patch fixes the issues for all four feeds.

See merge request !274