Why this change?
This is a follow-up to 86b2e3aa3e.
Basically, we want to allow people to select more than 1 category as well.
What does this change do?
1. Change `type: category` to `type: categories` and support `min` and `max`
validations for `type: categories`.
2. Fix the `<SchemaThemeSetting::Types::Categories>` component to support the
`min` and `max` validations and switch it to use the `<CategorySelector>` component
instead of the `<CategoryChooser>` component which only supports selecting one category.
Alan Guo Xiang Tan