Assign pull request to project during creation (go-gitea#28227)

When creating a pull request, allow directly assigning it to a project,
as it is already possible for issues.

After:


![grafik](https://github.com/go-gitea/gitea/assets/47871822/01dc2b3d-d56a-4053-b2fc-138725d7633a)

routers/web/repo/compare.go

@@ -845,6 +845,7 @@ func CompareDiff(ctx *context.Context) {
 		}
 	}
 
+	ctx.Data["IsProjectsEnabled"] = ctx.Repo.CanWrite(unit.TypeProjects)
 	ctx.Data["IsAttachmentEnabled"] = setting.Attachment.Enabled
 	upload.AddUploadContext(ctx, "comment")
 

routers/web/repo/pull.go

@@ -1387,7 +1387,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 		return
 	}
 
-	labelIDs, assigneeIDs, milestoneID, _ := ValidateRepoMetas(ctx, *form, true)
+	labelIDs, assigneeIDs, milestoneID, projectID := ValidateRepoMetas(ctx, *form, true)
 	if ctx.Written() {
 		return
 	}
@@ -1465,6 +1465,17 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 		return
 	}
 
+	if projectID > 0 {
+		if !ctx.Repo.CanWrite(unit.TypeProjects) {
+			ctx.Error(http.StatusBadRequest, "user hasn't the permission to write to projects")
+			return
+		}
+		if err := issues_model.ChangeProjectAssign(ctx, pullIssue, ctx.Doer, projectID); err != nil {
+			ctx.ServerError("ChangeProjectAssign", err)
+			return
+		}
+	}
+
 	log.Trace("Pull request created: %d/%d", repo.ID, pullIssue.ID)
 	ctx.JSONRedirect(pullIssue.Link())
 }