Hide question banks from MathJax

closes LS-1926
flag=new_math_equation_handling

test plan:
  - create a multiple choice question in a question bank
  - put inline LaTex in an answer
  - use the equation editor in another answer
  - save
  > expect to see the LaTex and not a mathjax typeset version
  > expect to see the equation image in the other answer
  - edit the question and answer and save
  > expect the same result as before
  - create a quiz. In the Questions tab click "Find a Question" and
    choose the question you created in the bank
  > expect the inline latex or eq. image when editing the question
  - save and preview the quiz
  > expect all the answers to be typeset

Change-Id: I534fe6a783da5c5a93eaa82493ffb8eeef181252
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/259294
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jeff Largent <jeff.largent@instructure.com>
QA-Review: Robin Kuss <rkuss@instructure.com>
Product-Review: Ed Schiebel <eschiebel@instructure.com>

app/views/question_banks/show.html.erb

@@ -79,7 +79,7 @@
     <span class="current_question_bank_id"><%= @bank.id %></span>
   </div>
 </div>
-<div id="questions" class="<%= 'uneditable' unless can_do(@bank, @current_user, :manage) %> question_editing brief question_bank show_correct_answers">
+<div id="questions" class="<%= 'uneditable' unless can_do(@bank, @current_user, :manage) %> question_editing brief question_bank show_correct_answers mathjax_ignore">
   <% if @questions.total_pages <= 1 %>
     <% @questions.each do |question| %>
       <%= render :partial => "quizzes/quizzes/display_question", :object => question.data, :locals => {:question_bank_id => @bank.id, :editing => true, :asset_string => question.asset_string} %>