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.

app/views/legislation/draft_versions/show.html.erb

@@ -66,7 +66,7 @@
                    data-legislation-annotatable-base-url="<%= legislation_process_draft_version_path(@process, @draft_version) %>"
                    data-legislation-open-phase="<%= @process.allegations_phase.open? %>">
           <% end %>
-            <%= sanitize(@draft_version.body_html) %>
+            <%= sanitize(@draft_version.body_html, { attributes: ["id"] }) %>
           </section>
         </div>
       </div>