Replace Ace Editor with ToastUi editor in the ExecutionEnvironments form

Drop unused code related to the Ace markdown editor.

app/views/execution_environments/_form.html.slim

@@ -1,4 +1,11 @@
-= form_for(@execution_environment) do |f|
+- content_for :head do
+  // Force a full page reload, see https://github.com/turbolinks/turbolinks/issues/326.
+     Otherwise, code might not be highlighted correctly (race condition)
+  meta name='turbolinks-visit-control' content='reload'
+  - append_javascript_pack_tag('toast-ui')
+  - append_stylesheet_pack_tag('toast-ui')
+
+= form_for(@execution_environment, builder: MarkdownFormBuilder) do |f|
   = render('shared/form_errors', object: @execution_environment)
   .mb-3
     = f.label(:name, class: 'form-label')
@@ -51,6 +58,5 @@
     = f.select(:testing_framework, @testing_framework_adapters, {include_blank: true}, class: 'form-control')
   .mb-3
     = f.label(:help, class: 'form-label')
-    = f.hidden_field(:help)
-    .form-control.markdown
+    = f.markdown :help
   .actions = render('shared/submit_button', f:, object: @execution_environment)