Avoid including ApplicationHelper in a controller

Because this would define all helper methods as action_methods. Instead,
let's call helper methods in view files as usual.

app/controllers/proposals_controller.rb

@@ -109,13 +109,12 @@ def update
     end
   end
 
-  include ApplicationHelper
   def parse_edit_field
     respond_to do |format|
       format.js do
         render locals: {
           field_name: params[:name],
-          text: markdown(params[:text])
+          text: params[:text]
         }
       end
     end

app/views/proposals/parse_edit_field.js.erb

@@ -1 +1 @@
-$('[data-field-name="<%= field_name %>"]')[0].innerHTML = '<%=j text %>';
+$('[data-field-name="<%= field_name %>"]')[0].innerHTML = '<%=j markdown(text) %>';