Integrate Traefik (2.0) to our stack

[elafarge]

I'm sorry, this commit contains a compilation of what I've added to
our Plex & co setup over the last two years, without commiting
anything. So... it actually contains multiple changes :/

I thought these changes would be beneficial to our pet project
Essentially, they are meant to make setup and maintainance easier than
ever before, which I think is essential if we want more people to enjoy
this stuff :D

Here's the list:

• Traefik has been integrated to our stack. It makes the setup easier
  for everyone who just wants a working setup on a fresh new server.
  Using this integrated traefik to route traffic to containers that
  aren't part of this stack is still possible of course.
  As a result, using custom networks is not necessary any more, it
  simplifies our docker-compose.yaml file a bit.
• I've updated to Traefik 2, which completely breaks the vast majority
  of its APIs. Docker annotations had to be rewritten.
• I removed the traefik.toml file and entirely moved the configuration
  to CLI flags. Removing this "moving part" that had to be edited and
  could be commited by accident (with our credentials in it...) will
  make "git pulls" much much easier... and therefore encourage
  contributions I hope :)
• The configuration just boils down to env. variables, which the README
  suggests should be put in the user's .bashrc/.zshrc (using
  docker-compose commands is therefore way simpler, no need to
  constantly use Ctrl-R any more).
• The plex token is no more passed as an env. var. It has to be entered
  on the UI on first setup.
• Only the HTTPs port is exposed, most browsers use https by default when
  typing URLs in the navigation bar. As a result, we now use the
  tlsChallenge (instead of http challenge): it doesn't require the
  server to listen on port 80. Only 443 is used during the ACME
  challenge.
• Some additional folders are mounted onto plex, they are targeted at
  manual uploads from other machines (I used that to RSync music from my
  multiple computers, as well as my Pictures, which I think is a rather
  valuable feature).

[aouaki]

Hey, thanks for your PR, the rework on the README was really needed.

I think you can add a small section in the troubleshooting part explaining how to update the images (to keep Plex & co. up-to-date)