Freshen up tracker branch

I merged it with upstream, also:

1) Fixed overlapping migrations.
2) Wrong content type (application/bittorrent instead of just binary).
3) Realized about a transmission client bug with magnets (https://trac.transmissionbt.com/ticket/5507).
4) Simplified view a bit.

Thanks!
Thanks, I've just reported it upstream in the aforementioned trac ticket, it's under moderation. Btw @nimiq, I'm looking at the REST API for post, trying to expose bittorrent/magnet links via json, cmdline client hopefully coming soon :)