Fix of broken Discourse.SiteSetting calls

Fix of `TypeError: Discourse.SiteSettings is undefined`  described at 
https://meta.discourse.org/t/mathjax-plugin-supports-math-notation-using-latex/12826/20
This error did not impact TeX rendering, but caused issues with other plugins such as Poll. 
Probably fixes kasperpeulen/discourse-mathjax#8 too.  
Additionally updated  how site settings are accessed to follow convention introduced in  https://github.com/discourse/discourse/commit/52094fe554338a0d8f6a009c93149bae09439a4a 

Tested with latest Discourse, enabling/disabling MathJax via site preference works as expected.