[API] times: use Index instead ID's and dont expose ID's

[6543]

While All Issue API Endpoints are structured like this:
/repos/{owner}/{repo}/issues/{index}/<endpoing> and use index
timetrack API is not:
/repos/{owner}/{repo}/issues/{id}/times

does this have any meanings?!?

[zeripath]

We also need to add the ability to adjust time tracking. @6543 I think there were a couple of issues that referenced this.

[noerw]

Another problem: the API returns internal IDs (user, issue ID), which is neither consistent with other APIs, nor meaningful to a data consumer. Should be username & issue index instead.

The following PR is blocked by this issue: https://gitea.com/gitea/tea/pulls/54