Add API to manage issue dependencies (#17935)

Add API to manage issue dependencies (#17935)

Adds API endpoints to manage issue/PR dependencies
* `GET /repos/{owner}/{repo}/issues/{index}/blocks` List issues that are
blocked by this issue
* `POST /repos/{owner}/{repo}/issues/{index}/blocks` Block the issue
given in the body by the issue in path
* `DELETE /repos/{owner}/{repo}/issues/{index}/blocks` Unblock the issue
given in the body by the issue in path
* `GET /repos/{owner}/{repo}/issues/{index}/dependencies` List an
issue's dependencies
* `POST /repos/{owner}/{repo}/issues/{index}/dependencies` Create a new
issue dependencies
* `DELETE /repos/{owner}/{repo}/issues/{index}/dependencies` Remove an
issue dependency

Closes https://github.com/go-gitea/gitea/issues/15393
Closes #22115

Co-authored-by: Andrew Thornton <art27@cantab.net>