Marking which instances use external services/trackers

Background: Many users of PeerTube have objections to trackers and also Google services (concrete evidence can be found in https://github.com/Chocobozzz/PeerTube/issues/1721).

Issue: Some PeerTube instances choose to include external services, including 2 of the 15 most popular:
_(simply looking at NoScript results, no in-depth checking was done)_
- **hostyour.tv** : popcash.net, pvclouds.com, waust.at (popcash.net is certainly a potentially unwanted site)
- **peertube.video** : googlesyndication.com, googletagmanager.com (Google services are controversial)

While I believe they have a right to do this (this is FLOSS, of course!), I also believe it is important that potential users are made aware. Maybe it would be good to add warnings on sites like https://instances.joinpeertube.org/instances to list potentially unwanted features.

Workaround: _For some sites, these trackers could be avoided by viewing videos using an alternative Peertube site which federates with the tracked site. However, you would of course need to first be able to discover which sites federate with it! (Is there a publicly available graph of Peertube instance federation? That would be a nice feature...)_

Start by reading the referenced issue #1721 and inspecting the instances listing at instances.joinpeertube.org/instances. The issue names no source files or tests and leaves the warning criteria and implementation undefined. Done would require an agreed scope for marking external services and a verifiable update to the listing.

Written by the indexing model from the issue text.