Add warnings to performance bar response

See merge request gitlab-org/gitlab-ce!31054

The original name has been deprecated

This key is useful to reduce the amount of logic needed on the frontend:
if `has_warnings` is true, then the frontend knows that the request in
question has warnings for some metric.

For each DetailedView subclass, we add a `warnings` array to:

1. The top-level response.
2. Each individual call under the `details` key.

We use the `.thresholds` hash on the DetailedView to determine what's a
warning. If that hash is empty (the default), then the warnings array
will always be empty.

A new param with_security_reports was added to
GET /groups/:id/projects API and the code to
support this logic in GroupProjectsFinder and
Project model. Also, a DB index was added to
ci_job_artifacts table to speed up the search
of security reports artifacts for projects

This also restructures how and where the configuration for
Snowplow lives.

Signed-off-by: Rémy Coutable <remy@rymai.me>

(cherry picked from commit 897a9d308db46b620b738b98f2b0e5630ac7d2dd)