Graphql: Add additional sort values for Issues and Issuables

Add additional sort values for Issues and Issuables, which can then be leveraged for Epics, such as for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14311

Already support:

• created_asc
• created_desc
• updated_asc
• updated_desc

Needed:

• due_date : !18094 (merged)
• relative_position : !19713 (merged)
• popularity : !14311 (merged)
• weight : !19721 (merged)

Broke out these out into separate issues due to their complexity:

• priority : #38293 (closed)
• label_priority : #38295 (closed)
• milestone_due : #38296 (closed)