Write a plugin that finds outdated attachments links in posts

[rafalp]

In #1813 I've changed attachment syntax in posts to <attachment=filename:id>, and implemented a simple markup updating migration that uses regexes to find and replace old syntax with new one.

There are limitations to what this implementation can accomplish, so I would still like to have a tool for site admins to manually finish the rest. And I would like to not have to keep it as part of Misago's codebase proper, so I could have a plugin that would add admin page for finding those posts for admin.

Simplest way to find attachments is to run Post.objects.filter(original__contains="/a/") search. This isn't too fast (especially on very big forums) but it does the job.