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/models/topic_link_click_spec.rb

@@ -145,6 +145,9 @@ describe TopicLinkClick do
 
           it "works with s3 urls" do
             SiteSetting.s3_cdn_url = "https://discourse-s3-cdn.global.ssl.fastly.net"
+            SiteSetting.s3_access_key_id = 'X'
+            SiteSetting.s3_secret_access_key = 'X'
+            SiteSetting.enable_s3_uploads = true
 
             post = Fabricate(:post, topic: @topic, raw: "[test](//test.localhost/uploads/default/my-test-link)")
             TopicLink.extract_from(post)