We have a native dependency, Rinku, that's used only to make links in one place. We can get rid of this and use PrettyText instead.
This is almost a one-for-one replacement, but PrettyText adds rel="noopener nofollow ugc" to external links, which I suspect is actually what we want. It also wraps the result in a <p> tag, which we strip out for parity with Rinku.
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.
