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/topic.rb

@@ -1433,7 +1433,7 @@ class Topic < ActiveRecord::Base
   end
 
   def apply_per_day_rate_limit_for(key, method_name)
-    RateLimiter.new(user, "#{key}-per-day", SiteSetting.send(method_name), 1.day.to_i)
+    RateLimiter.new(user, "#{key}-per-day", SiteSetting.get(method_name), 1.day.to_i)
   end
 
   def create_invite_notification!(target_user, notification_type, username)