Support asset links creation in `POST /projects/:id/release`

What does this MR do?

• Add Releases::Link and Releases::Source model, which belongs to Release model
• Add release_links table that persists name and url for each asset link
• Let POST /projects/:id/release create asset links. The full-fledged CURD operation is implemented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24100.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23795
• Related https://gitlab.com/gitlab-org/gitlab-ce/issues/41766

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
• [-] Tested in all supported browsers => This is not FE related changes
• 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