Add first-class releases API

What does this MR do?

This MR implements Releases API under /api/v4/projects/:id/releases

1. The old Release entity was renamed as TagRelease for backward compatibility with the old usage inside Tag entity
2. a new Release entity was provided with extended attributes
3. links assets (defined in see https://gitlab.com/gitlab-org/gitlab-ce/issues/41766) will be shipped with another MR
4. CreateReleaseService was extended in order to support tag creation when needed.
5. The old release management API under /api/v4/projects/:id/repository/tags/:tag_name/release was deprecated

What are the relevant issue numbers?

• see https://gitlab.com/gitlab-org/gitlab-ce/issues/41766

Follow-ups

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24056
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24057
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24058
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24059

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23901
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer