DEV: introduce new API to look up dynamic site setting

This removes all uses of both `send` and `public_send` from consumers of
SiteSetting and instead introduces a `get` helper for dynamic lookup

This leads to much cleaner and safer code long term as we are always explicit
to test that a site setting is really there before sending an arbitrary
string to the class

It also removes a couple of risky stubs from the auth provider test

app/models/embedding.rb

@@ -23,7 +23,7 @@ class Embedding < OpenStruct
 
   def save
     Embedding.settings.each do |s|
-      SiteSetting.send("#{s}=", send(s))
+      SiteSetting.set(s, public_send(s))
     end
     true
   rescue Discourse::InvalidParameters => p