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 commit adds the RailsMultisite middleware in test mode when Rails.configuration.multisite is true. This allows for much more realistic integration testing. The `multisite_spec.rb` file is rewritten to avoid needing to simulate a middleware stack.
Mixing multisite and standard specs can lead to issues (e.g. when using `fab!`)
Disabled the (upcoming https://github.com/discourse/rubocop-discourse/pull/11) rubocop rule for two files that have thoroughly tangled both types of specs.
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.