Add Web Interface and REST auth

[caco3]

xxx

This is adaption of #2241

TODO

• Test
• Provide webpage on denial
• Add documentation to doc repo
  • Note that it only uses basic auth thus it is sent as plain text, see https://stackoverflow.com/questions/9534602/what-is-the-difference-between-digest-and-basic-authentication. Digest ist not supported!
  • show user:password example
• Update changelog