EE-Port: Add all ApplicationSettings attributes to the API
This is a port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12851 to EE in which we reuse the attribute list so all attributes that can be set from the UI, can also be queried & set from the API.
This also includes https://gitlab.com/gitlab-org/gitlab-ce/commit/672a68d3724bcae676d18244c85566e7d664a169 to which this would conflict.
Edited by Robert Speicher