Mass Deleter: Include recent posts in fetch progress

[marcustyphoon]

Concept

As noted in #1595 (comment), when copying Mass Deleter's code into Mass Privater, I added a bit in order to update the progress counter of the post fetching based on the total number of fetched posts, ignoring the timestamp filter. Without this, someone with a huge post count who selected a long-ago timestamp would see the UI appear to freeze for a while.

Mass Deleter can, specifically in the case of deleting drafts where we allow filter-by-timestamp, have this added.