Display the number of files that are not indexed

[msm-code]

Suggested in #19

It will also be nice-to-have to show the number of files that aren't indexed because ls /mnt/samples | wc -l != num_of_indexex_files.

Needs backend changes, redesign, and pondering if it's even in scope. I can see how it can be useful though (for example, as an alert when autoindex job fails)

[msm-code]

Ok, after a month I still don't have a good idea how to do this efficiently and in a way that will work with s3, multiple agents, multiple ursadb instances, etc. Just counting the files is a very expensive task, so we can't just do it every request, or even every hour.

If that's important to anyone, feel free to reopen and we'll reconsider 🤔