Add issue triage policies for the CE/EE projects.

Fairly self-explanatory, this was moved from the GitLab CE project because it's meant primarily for internal use.

See these links for more information on the development of these policies:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/17693
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5026

cc: @stanhu @DouweM