This MR backports PrometheusMetric model to CE
and adds: common, identifier to figure out what kind of metric is used.

Add artifact information in job API

See merge request gitlab-org/gitlab-ce!21328

Make cache vs artifacts section more visible, and minor enhancements to text.

See merge request gitlab-org/gitlab-ce!21491

Resolve "Should we still use Flay?"

Closes #44750

See merge request gitlab-org/gitlab-ce!21476

Update installation and update docs for 11.3

See merge request gitlab-org/gitlab-ce!21481

Start validating against the exciting JSON schema for the job controller JSON
response.

The JSON schema is not complete since there is quite a lot missing, this will
be done in a separate issue.

gitlab-org/gitlab-ce#50101

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