Move features to core: "Related Issues"

Split the functionality of Related Issues into Simple Relation and Blocking Relation, and

1. Move Simple Relation to Core
2. Keep Blocking Relation in Starter.
3. 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 related_issues - e.g. blocked_issues. Update IssueLink's CreateService to check that blocked_issues is available if "blocking/blocked" link is being created. Perhaps also update REST APIs and controller to not accept "blocking/blocked" type when blocking_issues feature is not available.
• remove related_issues feature check from project's policy, at this point I think we can remove related_issues from 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_issues feature is available when creating new blocking relation).

Release Notes

A few months back, we announced we planned to open source 18 features. In honoring that commitment, Related Issues [1], Issues CSV Export, and Issue Board Focus Mode are now available in Core.

[1] This only encompasses "relates to" as "blocks" and "is blocked by" remains in a paid tier

https://docs.gitlab.com/ee/user/project/issues/related_issues.html#related-issues