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

script/import_scripts/base.rb

@@ -95,7 +95,7 @@ class ImportScripts::Base
     @site_settings_during_import = get_site_settings_for_import
 
     @site_settings_during_import.each do |key, value|
-      @old_site_settings[key] = SiteSetting.send(key)
+      @old_site_settings[key] = SiteSetting.get(key)
       SiteSetting.set(key, value)
     end
 
@@ -108,7 +108,7 @@ class ImportScripts::Base
 
   def reset_site_settings
     @old_site_settings.each do |key, value|
-      current_value = SiteSetting.send(key)
+      current_value = SiteSetting.get(key)
       SiteSetting.set(key, value) unless current_value != @site_settings_during_import[key]
     end