Permission check for every view in Datasette (plus docs)

[simonw]

Every view in Datasette should perform a permission check to see if the current user/actor is allowed to view that page.

This permission check will default to allowed, but having this check will allow plugins to lock down access selectively or even to everything in a Datasette instance.

[simonw]

I'm going to need to add permissions documentation for this.

[simonw]

... and I want a unit test which confirms that all permissions are documented.