DEV: Clean up S3 specs, stubs, and helpers

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.

spec/requests/static_controller_spec.rb

@@ -52,9 +52,7 @@ describe StaticController do
       end
 
       before do
-        SiteSetting.enable_s3_uploads = true
-        SiteSetting.s3_access_key_id = 'X'
-        SiteSetting.s3_secret_access_key = 'X'
+        setup_s3
       end
 
       it 'can proxy a favicon correctly' do