Add feature flags for enabling (Upload|Receive)Pack from CE
In order to merge https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/143 without breaking anything (see https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/143#note_26500330 and https://gitlab.com/gitlab-org/gitlab-ce/issues/29925), we need to control when these features can be used with flags.