FEATURE: allow specifying s3 config via globals

This refactors handling of s3 so it can be specified via GlobalSetting

This means that in a multisite environment you can configure s3 uploads
without actual sites knowing credentials in s3

It is a critical setting for situations where assets are mirrored to s3.

spec/components/stylesheet/importer_spec.rb

@@ -20,8 +20,13 @@ describe Stylesheet::Importer do
   end
 
   it "applies S3 CDN to background category images" do
+    SiteSetting.s3_use_iam_profile = true
+    SiteSetting.s3_upload_bucket = 'test'
+    SiteSetting.s3_region = 'ap-southeast-2'
     SiteSetting.s3_cdn_url = "https://s3.cdn"
 
+    SiteSetting.enable_s3_uploads = true
+
     background = Fabricate(:upload_s3)
     category = Fabricate(:category, uploaded_background: background)