fix editing on non-external markdown levels

[davidsbailey]

editing of multi levels was reported here: https://codedotorg.slack.com/archives/CA3KCSGTD/p1603994532318000 the page does not navigate after clicking save, but the edits made appear persist:

This PR fixes the regression, which was introduced in https://github.com/code-dot-org/code-dot-org/pull/35683/files#diff-eefd248014f6b9740cc3067f810e31bb979e3369f3d2abf425f93e3af759229bR35 , by fixing the parts of that PR that broke things. That PR wiped out the editor components needed for markdown editor fields on DSL-defined levels. without those fields, the initialization code failed:

This caused the ajax-handling code here never to execute:

code-dot-org/apps/src/code-studio/ajaxSubmit.js

Lines 3 to 5 in 1cad5ac

	module.exports = function ajaxSubmit(form_selector) {
	$(document).ready(function() {
	$(form_selector).on('ajax:beforeSend', function(e, xhr) {

presumably leading to the navigation not happening after save.

Testing story

I will follow up with an attempt to add a UI test to cover saving of a multi level.

[bethanyaconnor]

Sorry for breaking and thanks for fixing!