Allow adding reviewers for issues

### Feature Description

Commonly in my usage of Gitea I find myself wishing that Gitea had an ability to differentiate regular "assignees" from "reviewers" of the issue. Commonly reviewer is the person that created the issue, but not always.

This will also simplify filtering of the issues, allowing to differentiate between issues that you have to implement vs issues that you have to review.

### Screenshots


The issue names no files, tests, or entry points, so start by tracing how Gitea currently stores and displays issue assignees and how issue filtering works. Define the reviewer model and its UI and filtering behavior before implementation. Done means reviewers are distinct from assignees and issues can be filtered by those roles.

Written by the indexing model from the issue text.