[feature] add a project page to list all open issues

[hsluoyz]

It will be like: https://github.com/orgs/casbin/projects/2 but it's fully automatic.

• List all open issues (and relevant PRs if any). If issue or PR is closed (or merged), it will be removed.
• Multiple categories like: https://github.com/orgs/casbin/projects/2 , Casbin, Casdoor, jCasbin, etc.
• Issue can be assigned to a github user. The issues are sorted by the assign time.

[casbin-bot]

@hsluoyz