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/policies/post_policy.rb

@@ -31,6 +31,10 @@ class PostPolicy < ApplicationPolicy
     user.is_approver? && !record.is_deleted?
   end
 
+  def destroy?
+    delete?
+  end
+
   def ban?
     user.is_approver? && !record.is_banned?
   end