Show whether a PR is WIP inside popups (#28975)

Fixes https://codeberg.org/forgejo/forgejo/issues/2257

Draft status of a PR is currently not exposed by the API. This PR adds a
'draft' field to pull requests in the API, which is used to correctly
set the PR color/icon in a ContextPopup.

---

Before:

![image](https://github.com/go-gitea/gitea/assets/5541521/72cbd30e-1175-4338-aa97-ac99c46c5118)

After:

![image](https://github.com/go-gitea/gitea/assets/5541521/111c9eba-460e-4d57-bcca-23a151c3a4f1)

modules/structs/issue.go

@@ -26,8 +26,9 @@ const (
 
 // PullRequestMeta PR info if an issue is a PR
 type PullRequestMeta struct {
-	HasMerged bool       `json:"merged"`
-	Merged    *time.Time `json:"merged_at"`
+	HasMerged        bool       `json:"merged"`
+	Merged           *time.Time `json:"merged_at"`
+	IsWorkInProgress bool       `json:"draft"`
 }
 
 // RepositoryMeta basic repository information