Filter by category should require a page type

[richaagarwal]

Currently, you can select "Filter category", but not a "Page type", as shown:

and successfully save the page, but it will error out when you try to view the page because there's no page type. We should add validation here.

[chosak]

This no longer errors when you view the page, but it shows up as an empty category list:

We could add validation to ensure that users have to select a list of categories if they want to filter by category; a simpler UI would probably be to only show the dropdown, and hide the category filter unless an option was selected.