Use Entry title to set downloaded file name

Instead of using the MD5 hash of the enclosure download URL, we
create a filename which follows `feedname/entry_title.hash.ext`, where
feedname is a uniquefied feed title (stored in the DB), a truncated version
of the entry title, a shortened hash based on the download URL, and the
original file extension extracted from the download URL.

BUG: 457848