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!
No problem, done: https://github.com/brainstorm/biostar-central/commit/95afd5543e281bff7

With `info_hash` or `magnet_url` one can fetch torrents already... I hope @ialbert merges this branch, it's looking good :)