1. Use `category.url` instead instead of forming URLs by hand. The
subcategory urls were wrong but the tests were passing since we were
asserting for the component not displaying which will still pass when
the page 404s.
1. Use `let!` instead of `fab!` for `upload_theme_component` since `let!` ensures that the theme cache is cleared between tests.
This commits introduces the follow changes:
1. Use the `renderInOutlet` API over the connectors convention
1. Renames `discourse-category-banners` component to `category-banner`
which is more appropriate cause we are only rendering banner for one
category.
1. Adds system tests
