posts: rework post deletion to use dialog box.

Rework post deletion from using a separate page to using a dialog box,
like flagging.

* Add `DELETE /posts/:id` endpoint.
* Remove `POST /moderator/post/posts/:id/delete` endpoint.

app/views/posts/partials/show/_delete_dialog.html.erb

@@ -0,0 +1,9 @@
+<div class="delete-post-dialog-body">
+  <%= edit_form_for(post, method: :delete, remote: true) do |f| %>
+    <input type="hidden" name="commit" value="Delete">
+    <%= f.input :reason, as: :dtext, inline: true, input_html: { value: "" } %>
+    <% if post.parent_id.present? %>
+      <%= f.input :move_favorites, label: "Move favorites to parent", as: :boolean, input_html: { checked: false } %>
+    <% end %>
+  <% end %>
+</div>