Currently, the light version of mobile logo falls back to the desktop
version if the mobile version isn't set. It makes sense to have the same
fallback rule for the dark version as well, i.e. if there's no dark
mobile logo, use the dark desktop logo.
Internal topic: t/150316.
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.