* Added can_see_bookmarkable? methods to BaseBookmarkable, implement
that in QueryGroupBookmarkable
* Update spec to check that Notification for reminder has
bookmarkable_id and bookmarkable_type
c.f. https://github.com/discourse/discourse/pull/25905
This commit updates the plugin to the latest guidelines, as shown in
discourse-plugin-skeleton, which involves moving a lot of the code to
dedicated files, use proper namespaces, use the autoloader as much as
possible, etc.