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!
The posts endpoint is here:
https://github.com/nimiq/biostar-central/blob/api/biostar/apps/posts/api_serializers.py#L111
It should be easy to add a new field for the magnet link.

It's in the `api` branch of my repo, not merged upstream yet cause doc and tests are missing. I will pick this up again when I have some free time, hopefully by the end of the year.