Add setting for role access

[frankiejarrett]

We should allow Stream access to be granted to any user role.

However, I think only site Administrators (and Super Admins for multisite) should be able to access the Stream Settings screen.

[kucrut]

@fjarrett If we were to do this, then we also need to modify the link(s) under the action summaries. For example, Contributors won't be able to view the Settings > General page that's currently always shown under the summary when something changed there.

[kucrut]

@fjarrett Also, since add_menu_page() and friends requires a capability argument and not role, I think we need to use a custom capability when registering the parent menu and filter user_has_cap to dynamically grant capability to users from the allowed roles.