Archiving byzcoin

[ineiti]

Archiving of the byzcoin service:

• creating docker images with a pre-initialized blockchain
• include omniledger, ol-explorer, and columbus-united as webapps
• add a landing page with explanations

Code review steps:

• follow README.md to start the demo
• go through the signup and test the other two demos
• give feedback/create PRs on:
  • text itself (grammar, understandability)
  • how easy it is to follow the instructions
  • the description of the Troubleshooting part
• regarding the code, only point out big mistakes that really should be cleaned up before merging. I don't expect you to go through all files added / modified, especially not those in pkg*, which are mostly copied from cothority.

Closes c4dt/TODO#324

[PascalinDe]

works locally, LGTM