Search support for epics

Description

Currently, GitLab lacks any kind of support for searching epics from the /search page. This means that we can't search globally for epics, and we can't enhance those searches with elasticsearch when customers have that enabled and configured too.

Proposal

Add support for epics to Search::GlobalService, Search::GroupService and (once https://gitlab.com/gitlab-org/gitlab-ee/issues/4019 is finished, or as part of it), Search::ProjectService.

We should also ensure that elasticsearch can index epics, which will involve adding a new document type (better to be a brand-new separate index since document type support is going away in ES 6). Since we'll be indexing by group id, rather than project id, this will be more natural anyway.

Links / references

/cc @victorwu