Searching for snippets with elasticsearch enabled still looks in the database

Summary

Elasticsearch indexes snippets, but search queries with elasticsearch enabled always go directly to the database, rather than via ES :(

Steps to reproduce

Do a search. Note that 0 time is spent in elasticsearchin the rails logs.

Also note that Gitlab::Elastic::SnippetSearchResults is defined in the codebase, but never used.

What is the current bug behavior?

Queries the database

What is the expected correct behavior?

Should query Elasticsearch

Possible fixes

Search::SnippetsService needs modifying in EE to call Gitlab::Elastic::SnippetSearchResults

More generally, it might be a good idea to merge the snippet search service into the global, group, and project search services, for a more unified interface. In the global search, we'd include all snippet types; in group and project search, only project snippets in the specified group or project would be included.

Naive fix partway in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10325/diffs