Add AssumeUTXO snapshot

[Sjors]

This adds a magnet link and brief instructions to the bottom of the Download page.

AssumeUTXO Snapshot

The following UTXO snapshot can be used to quickly sync from block 910000 to the tip. Use the loadtxoutset RPC to load it.
• Torrent magnet: 🧲

It's just a magnet link right now, but I can add a "Download torrent" link if we commit to hosting the .torrent file.

Fixes #1130

[l0rinc]

My understanding is that PR descriptions shouldn't contain images (misbehaves with merge scripts or something), maybe we could replace it with the actual text:

AssumeUTXO Snapshot

The following UTXO snapshot can be used to quickly sync from block 910000 to the tip. Use the loadtxoutset RPC to load it.

• Torrent magnet: 🧲

[jaonoctus]

tACK.

I see that you've included the others trackers that I was also using like udp://tracker.opentrackr.org:1337/announce.

[Sjors]

@jaonoctus they should be the same trackers we use for releases.