Rename PagedownFormBuilder to MarkdownFormBuilder

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

app/views/tips/_form.html.slim

@@ -5,14 +5,14 @@
   - append_javascript_pack_tag('toast-ui')
   - append_stylesheet_pack_tag('toast-ui')
 
-= form_for(@tip, builder: PagedownFormBuilder) do |f|
+= form_for(@tip, builder: MarkdownFormBuilder) do |f|
   = render('shared/form_errors', object: @tip)
   .mb-3
     = f.label(:title, class: 'form-label')
     = f.text_field(:title, class: 'form-control', required: false)
   .mb-3
     = f.label(:description, class: 'form-label')
-    = f.pagedown :description
+    = f.markdown :description
   .mb-3
     = f.label(:file_type_id, t('activerecord.attributes.file.file_type_id'), class: 'form-label')
     = f.collection_select(:file_type_id, @file_types, :id, :name, {include_blank: true}, class: 'form-control')