Allow PRs to be marked as WIP (thus preventing them to be accidentally merged)

[gergelypolonkai]

One thing I really miss from GitLab (I think this feature is non-existent on GitHub) is the ability to mark a pull request as WIP (Work in Progress). All GitLab MRs whose title begins with WIP: is blocked from being merged.

I don’t know if this would fit the plans for Gitea, but it could be definitely helpful for some.

[JulienTant]

Hi, i'm willing to try to implement this in gitea, but before I wanna make sure that you're not working on it on your side.

I imagine building this in two PRs :

1. Disable to ability to merge a PR when the title starts by WIP: (maybe we can make this configurable ?)
2. An hypotetical second PR to add "helpers" like :

• "When the title starts with WIP:, add a button "remove WIP status" that edits the title to remove WIP:
• When we create a PR, add a description with a link like "Add WIP: to the title to prevent this PR to be accidentally merged"