Add limits for an epics' issues and child epics

As discussed in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9283#note_139008986, the query for an epics' issues and child epics do not have a LIMIT.

The list of children could get arbitrarily large and may cause performance issues in the future.

I opened this issue for discussion. Should we enforce a hard limit on the number of child epics / issues? Or should we support pagination in the UI for this?

Acceptance Criteria

• Limit of 2000 is applied to issues and sub-epics of any given epic;
• Proposed limit of 2000 is validated to ensure that it doesn't affect any existing customers, and
• Limited is documented on https://docs.gitlab.com/ee/administration/instance_limits.html