These factories were only used in one place and they even declared ID
attributes. Using the comment factory with the `commentable` attribute
does the same thing.
The `type: :feature` is automatically detected by RSpec because these
tests are inside the `spec/features` folder. Using `feature` re-adds a
`type: :feature` to these files, which will result in a conflict when we
upgrade to Rails 5.1's system tests.
Because of this change, we also need to change `background` to `before`
or else these tests will fail.
We're also adding a rubocop rule so we dont' accidentally add these
keywords again.
