The most common thing that we do with fab! is:
fab!(:thing) { Fabricate(:thing) }
This commit adds a shorthand for this which is just simply:
fab!(:thing)
i.e. If you omit the block, then, by default, you'll get a `Fabricate`d object using the fabricator of the same name.
The site.json endpoint was missing some optional fields on the
categories property that is returned. This commit documents the
`parent_category_id` which is present if there are subcategories and it
also documents the `custom_fields` property which the chat plugin is
using causing some flaky tests.
