Move features to core: "Related Issues"
Split the functionality of Related Issues into Simple Relation and Blocking Relation, and
- Move Simple Relation to Core
- Keep Blocking Relation in Starter.
- Update E2E test (#212329 (comment 315423065))
BE steps might be:
- add a new licensed feature representing "blocked issues" feature to ee/app/models/license.rb next to
blocked_issues. Update IssueLink's CreateService to check that
blocked_issuesis available if "blocking/blocked" link is being created. Perhaps also update REST APIs and controller to not accept "blocking/blocked" type when
blocking_issuesfeature is not available.
- remove related_issues feature check from project's policy, at this point I think we can remove
related_issuesfrom ee/app/models/license.rb too because we don't use it anywhere else. Move all Related issues code to core (except EE-override for checking that
blocking_issuesfeature is available when creating new blocking relation).
 This only encompasses "relates to" as "blocks" and "is blocked by" remains in a paid tier