Add container.FilterSlice function (gitea#30339)

Many places have the following logic:
```go
func (jobs ActionJobList) GetRunIDs() []int64 {
	ids := make(container.Set[int64], len(jobs))
	for _, j := range jobs {
		if j.RunID == 0 {
			continue
		}
		ids.Add(j.RunID)
	}
	return ids.Values()
}
```

this introduces a `container.FilterMapUnique` function, which reduces
the code above to:
```go
func (jobs ActionJobList) GetRunIDs() []int64 {
	return container.FilterMapUnique(jobs, func(j *ActionRunJob) (int64, bool) {
		return j.RunID, j.RunID != 0
	})
}
```
Conflicts:
models/issues/comment_list.go due to premature refactor in #3116

models/repo/repo_list.go

@@ -104,18 +104,19 @@ func (repos RepositoryList) LoadAttributes(ctx context.Context) error {
 		return nil
 	}
 
-	set := make(container.Set[int64])
+	userIDs := container.FilterSlice(repos, func(repo *Repository) (int64, bool) {
+		return repo.OwnerID, true
+	})
 	repoIDs := make([]int64, len(repos))
 	for i := range repos {
-		set.Add(repos[i].OwnerID)
 		repoIDs[i] = repos[i].ID
 	}
 
 	// Load owners.
-	users := make(map[int64]*user_model.User, len(set))
+	users := make(map[int64]*user_model.User, len(userIDs))
 	if err := db.GetEngine(ctx).
 		Where("id > 0").
-		In("id", set.Values()).
+		In("id", userIDs).
 		Find(&users); err != nil {
 		return fmt.Errorf("find users: %w", err)
 	}