New pattern for views that return either JSON or HTML, available for plugins

[simonw]

Can be part of #870 - refactoring existing views to use register_routes().

I'm going to put the new check_permissions() method on BaseView as well. If I want that method to be available to plugins I can do so by turning that BaseView class into a documented API that plugins are encouraged to use themselves.
Originally posted by @simonw in #832 (comment)

[simonw]

The BaseView class does this for Datasette internals at the moment, but I'm not convinced it works as well as it could.

I'd like to turn this into a class that is documented and available to plugins as well.

[simonw]

I think this is blocking #619

[simonw]

This has been blocking things for too long.

If this becomes a documented pattern, things like adding a JSON output to https://github.com/dogsheep/dogsheep-beta becomes easier too.