[REF] ir_qweb: less webasset: compress output instead of minifying it

* Remove dependency on less-plugin-clean-css which distribution
  packages are broken on debian stretch and derivated.
* Updated documentation.
* Lessc provides a `--compress` argument that could solve this issue.
  According to the documentation at http://lesscss.org/usage/: "[...]
  Compress using less built-in compression. This does an okay job but
  does not utilise all the tricks of dedicated css compression[...]"
  However, using this argument works on lessc 2.5.3 but not on 1.4.2
  (`--compress` arg is present but seems to be break some rules
  i haven't been able to put my finger on).
* Finally, use the minification of the StylesheetAsset that takes care
  of removing the spaces, the comments and the sourcemap with regexes.

fixes issue #9113