Added a content field to atom feed

What does this MR do?

This addresses issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/13453

Adds a content field in addition to the description field for atom feeds. The RSS spec uses description, but these feeds are specified as atom and should adhere to the Atom standard.

The Atom spec does allow for extra tags which are not part of the spec (such as labels and assignee), but readers are unlikely to read them. This is an easy solution that should avoid breaking anything and just duplicates the description as content.

This MR is an alternative to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27650 and does not provide XML that conforms to the atom spec, but also doesn't carry the risks of the other MR. Only one or the other of these should be implemented.