Support ordering by weight for /api/v4/issues

Description

GitLab EE has issue weights. In the frontend, issues can be sorted by weight, but there is no provision for this in the API:

$ curl 'http://localhost:4000/api/v4/issues?order_by=weight&sort=desc&private_token=...'
{"error":"order_by does not have a valid value"}

Proposal

Add the ability to sort issues by weight in GitLab EE