Invite only and Discourse connect could not be enabled at the same time
because of some legacy reason. This is a follow up commit to ce04db8,
355d51a and 40f6ceb.
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.