Consider removing expensive "subscribed?" field from responses that return issues list

GET /api/projects/:id/epics/:epic_iid/issues or /api/:version/projects/:id/issues/:issue_iid/links expose subscribed? field, that tells whether a user is subscribed to an issue or note.

In order to calculate this field, we look for subscriptions or iterates over participants. As a result, O(N) queries for N issues are performed.

Proposal

Deprecate and remove subscribed? field for issues list.

For Issues API we don't return this field, but we return this field for issues/:id request. Participants are not cheap to calculate, so we'd rather allow fetching them for a single issue.