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

lib/validators/upload_validator.rb

@@ -123,7 +123,7 @@ class Validators::UploadValidator < ActiveModel::Validator
     max_size_kb = if upload.for_export
       SiteSetting.max_export_file_size_kb
     else
-      SiteSetting.send("max_#{type}_size_kb")
+      SiteSetting.get("max_#{type}_size_kb")
     end
     max_size_bytes = max_size_kb.kilobytes