Workaround bug 1624550 by emk

Firefox 73 or later has a bug where some download links are not colored correctly iff the links redirect to actual download locations:
https://bugzilla.mozilla.org/show_bug.cgi?id=1624550
This add-on workarounds the issue by adding a history entry for the download URL.

This add-on requires following permissions:

• webReuest: to tell the pre-redirect URL.
• history: to manupilate history entries.
• download: to be notified when downoad complleted.