Allow per-page user access control with `allowed-users` setting

### Description

> What problem would this feature solve?

I'd love to share my Glance instance with people but it has a private page.

It would be great if we could configure a list of allowed users for each page.

> If applicable, what would the configuration for this feature look like?

```yaml
pages:
- name: Private
allowed-users:
- admin
- friend
columns: ...
```