Add "self" link to Release API response

What does this MR do?

Adds a new property, _links.self, to the GET /projects/:id/releases and GET /projects/:id/releases/:tag_name Release API endpoints. This link points to the dedicated page for the Release item being implemented in #32827 (closed).

This follows the example of the Projects API, which includes a _links.self property.

Feature flag

This new property is hidden by the release_show_page feature flag, which is disabled by default.

Dependencies

This MR is dependent on changes in !23792 (merged), and this MR's pipeline won't pass until !23792 (merged) is merged.