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)

services/convert/issue.go

@@ -98,7 +98,8 @@ func toIssue(ctx context.Context, issue *issues_model.Issue, getDownloadURL func
 		}
 		if issue.PullRequest != nil {
 			apiIssue.PullRequest = &api.PullRequestMeta{
-				HasMerged: issue.PullRequest.HasMerged,
+				HasMerged:        issue.PullRequest.HasMerged,
+				IsWorkInProgress: issue.PullRequest.IsWorkInProgress(ctx),
 			}
 			if issue.PullRequest.HasMerged {
 				apiIssue.PullRequest.Merged = issue.PullRequest.MergedUnix.AsTimePtr()