Rename PagedownFormBuilder to MarkdownFormBuilder

The term 'Pagedown' was originally associated with the pagedown-bootstrap
library, which is no longer in use.

app/helpers/markdown_form_builder.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class MarkdownFormBuilder < ActionView::Helpers::FormBuilder
+  def markdown(method, args = {})
+    # Adopt simple form builder to work with form_for
+    @attribute_name = method
+    @input_html_options = args[:input_html]
+
+    @template.capture do
+      @template.concat form_textarea
+      @template.concat @template.tag.div(class: 'markdown-editor', data: {behavior: 'markdown-editor-widget', id: label_target})
+    end
+  end
+
+  private
+
+  def form_textarea
+    @template.text_area @object_name, @attribute_name,
+    **(@input_html_options || {}),
+      id: label_target,
+      class: 'd-none'
+  end
+
+  def base_id
+    options[:markdown_id_suffix] || @attribute_name
+  end
+
+  def label_target
+    "markdown-input-#{base_id}"
+  end
+end