Provide redundancy for a single user

[Jorropo]

• PeerTube version or URL:
  peertube.jorropo.ovh
  beta.16

It could be good to choose to make a redundancy of a single user (somethings like strategy: 'user:[user]@[instances]')

[Nutomic]

It would probably make more sense to add a setting to the users list, just how it works with instances. Then users can be treated the same way as instances (with different strategies). The UI could also be moved into a seperate tab.