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.
Extracted commonly used spec helpers into spec/support/uploads_helpers.rb, removed unused stubs and let definitions. Makes it easier to write new S3-related specs without copy and pasting setup steps from other specs.
