remove mathjax form main xmodule requirement and add it on problem addin...

...g

TNL-823
@waheedahmed @mlkwaqas @adampalay 
I'm still nervous if this will work properly when part of the LMS is using requireJS (per the PR that is about to merge). We need to make sure that the RequireJS config there exports mathjax on the window object. I'd prefer to hold on this PR until #6158 merges, and then update the require config if necessary.