It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
This is a feature that used to be present in discourse-assign but is
much easier to implement in core. It also allows a topic to be assigned
without it claiming for review and vice versa and allows it to work with
category group reviewers.
