Provide API for project topics

[slonopotamus]

• Gitea version: 1.9.0 (actually, any)

Description

As far as I know, topics are currently totally unavailable through Gitea API.

What is needed:

• API to get existing topics as part of project info
• API to add new topics to project
• API to remove existing topics from project

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 weeks. Thank you for your contributions.

[zeripath]

Fixed by #7963