Convert markdown to HTML on demand

We were converting markdown to HTML every time we saved a record, which
has the same problems as sanitizing HTML before saving it to the
database, particularly because the body of a legislation draft is stored
in a translations table.

Performance-wise this isn't a problem: converting a text with more than
200_000 characters takes about a milisecond on my machine.

Note we need to modify a migration generated by globalize, since the
method `create_translation_table!` would fail now that we don't define
`translates :body_html` in the model.

db/migrate/20180801140800_add_collaborative_legislation_translations.rb

@@ -20,12 +20,14 @@ class AddCollaborativeLegislationTranslations < ActiveRecord::Migration[4.2]
       {
         title:     :string,
         changelog: :text,
-        body:      :text,
-        body_html: :text,
-        toc_html:  :text
+        body:      :text
       },
       { migrate_data: true }
     )
+
+    add_column :legislation_draft_version_translations, :body_html, :text
+    add_column :legislation_draft_version_translations, :toc_html, :text
+
     Legislation::QuestionOption.create_translation_table!(
       { value: :string },
       { migrate_data: true }

db/migrate/20191006114603_remove_html_fields_form_legislation_draft_version.rb

@@ -0,0 +1,8 @@
+class RemoveHtmlFieldsFormLegislationDraftVersion < ActiveRecord::Migration[5.0]
+  def change
+    remove_column :legislation_draft_versions, :body_html
+    remove_column :legislation_draft_version_translations, :body_html
+    remove_column :legislation_draft_versions, :toc_html
+    remove_column :legislation_draft_version_translations, :toc_html
+  end
+end