Preload associations and issuable metadata in Epics

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25711#note_147728908 will eliminate N+1 queries for Todos, but it needs to be adapted for Epics:

1. Add a test case for Todos
2. Add with_api_entity_associations to the Epics model
3. Deal with epics_metadata option (e.g. rename to issuable_metdata)